diff --git a/fixtures/dom/.gitignore b/fixtures/dom/.gitignore
index 724d1459422c9..3473f8a18efa5 100644
--- a/fixtures/dom/.gitignore
+++ b/fixtures/dom/.gitignore
@@ -8,6 +8,8 @@ coverage
 
 # production
 build
+public/scheduler-unstable_mock.development.js
+public/scheduler-unstable_mock.production.min.js
 public/react.development.js
 public/react.production.min.js
 public/react-dom.development.js
diff --git a/fixtures/dom/package.json b/fixtures/dom/package.json
index f5f84257f7c82..3282c85fa58a7 100644
--- a/fixtures/dom/package.json
+++ b/fixtures/dom/package.json
@@ -18,7 +18,7 @@
   },
   "scripts": {
     "start": "react-scripts start",
-    "prestart": "cp ../../build/node_modules/react/umd/react.development.js ../../build/node_modules/react-dom/umd/react-dom.development.js ../../build/node_modules/react/umd/react.production.min.js ../../build/node_modules/react-dom/umd/react-dom.production.min.js ../../build/node_modules/react-dom/umd/react-dom-server.browser.development.js ../../build/node_modules/react-dom/umd/react-dom-server.browser.production.min.js ../../build/node_modules/react-dom/umd/react-dom-test-utils.development.js ../../build/node_modules/react-dom/umd/react-dom-test-utils.production.min.js public/",
+    "prestart": "cp ../../build/node_modules/scheduler/umd/scheduler-unstable_mock.development.js ../../build/node_modules/scheduler/umd/scheduler-unstable_mock.production.min.js ../../build/node_modules/react/umd/react.development.js ../../build/node_modules/react-dom/umd/react-dom.development.js ../../build/node_modules/react/umd/react.production.min.js ../../build/node_modules/react-dom/umd/react-dom.production.min.js ../../build/node_modules/react-dom/umd/react-dom-server.browser.development.js ../../build/node_modules/react-dom/umd/react-dom-server.browser.production.min.js ../../build/node_modules/react-dom/umd/react-dom-test-utils.development.js ../../build/node_modules/react-dom/umd/react-dom-test-utils.production.min.js public/",
     "build": "react-scripts build && cp build/index.html build/200.html",
     "test": "react-scripts test --env=jsdom",
     "eject": "react-scripts eject"
diff --git a/fixtures/dom/public/act-dom.html b/fixtures/dom/public/act-dom.html
index 2fb4a437721df..2100026a5ba45 100644
--- a/fixtures/dom/public/act-dom.html
+++ b/fixtures/dom/public/act-dom.html
@@ -7,7 +7,11 @@
   this page tests whether act runs properly in a browser.
   <br/>
   your console should say "5"
+  <script src='scheduler-unstable_mock.development.js'></script>
   <script src='react.development.js'></script>
+  <script type="text/javascript">    
+    window.React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED.Scheduler = window.SchedulerMock
+  </script>
   <script src='react-dom.development.js'></script>
   <script src='react-dom-test-utils.development.js'></script>
   <script>
diff --git a/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.internal.js b/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.internal.js
new file mode 100644
index 0000000000000..d629795c0727e
--- /dev/null
+++ b/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.internal.js
@@ -0,0 +1,28 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+let ReactFeatureFlags;
+let act;
+describe('mocked scheduler', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    ReactFeatureFlags = require('shared/ReactFeatureFlags');
+    ReactFeatureFlags.warnAboutMissingMockScheduler = true;
+    jest.unmock('scheduler');
+    act = require('react-dom/test-utils').act;
+  });
+  it("should warn when the scheduler isn't mocked", () => {
+    expect(() => act(() => {})).toWarnDev(
+      [
+        'Starting from React v17, the "scheduler" module will need to be mocked',
+      ],
+      {withoutStack: true},
+    );
+  });
+});
diff --git a/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js b/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js
index 9969fa7211252..fd32092584d06 100644
--- a/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js
+++ b/packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js
@@ -11,6 +11,7 @@ let React;
 let ReactDOM;
 let ReactTestUtils;
 let SchedulerTracing;
+let Scheduler;
 let act;
 let container;
 
@@ -25,482 +26,490 @@ function sleep(period) {
 }
 
 describe('ReactTestUtils.act()', () => {
-  beforeEach(() => {
-    jest.resetModules();
-    React = require('react');
-    ReactDOM = require('react-dom');
-    ReactTestUtils = require('react-dom/test-utils');
-    SchedulerTracing = require('scheduler/tracing');
-    act = ReactTestUtils.act;
-    container = document.createElement('div');
-    document.body.appendChild(container);
-  });
-  afterEach(() => {
-    ReactDOM.unmountComponentAtNode(container);
-    document.body.removeChild(container);
-  });
-
-  describe('sync', () => {
-    it('can use act to flush effects', () => {
-      function App(props) {
-        React.useEffect(props.callback);
-        return null;
-      }
-
-      let calledLog = [];
-      act(() => {
-        ReactDOM.render(
-          <App
-            callback={() => {
-              calledLog.push(calledLog.length);
-            }}
-          />,
-          document.createElement('div'),
-        );
-      });
+  // first we run all the tests with concurrent mode
+  let concurrentRoot;
+  function renderConcurrent(el, dom) {
+    concurrentRoot = ReactDOM.unstable_createRoot(dom);
+    concurrentRoot.render(el);
+  }
+  function unmountConcurrent(_dom) {
+    if (concurrentRoot !== null) {
+      concurrentRoot.unmount();
+      concurrentRoot = null;
+    }
+  }
+  runActTests('concurrent mode', renderConcurrent, unmountConcurrent);
+
+  // and then in sync mode
+  function renderSync(el, dom) {
+    ReactDOM.render(el, dom);
+  }
+  function unmountSync(dom) {
+    ReactDOM.unmountComponentAtNode(dom);
+  }
+  runActTests('legacy sync mode', renderSync, unmountSync);
+});
 
-      expect(calledLog).toEqual([0]);
+function runActTests(label, render, unmount) {
+  describe(label, () => {
+    beforeEach(() => {
+      jest.resetModules();
+      React = require('react');
+      ReactDOM = require('react-dom');
+      ReactTestUtils = require('react-dom/test-utils');
+      SchedulerTracing = require('scheduler/tracing');
+      Scheduler = require('scheduler');
+      act = ReactTestUtils.act;
+      container = document.createElement('div');
+      document.body.appendChild(container);
     });
-
-    it('flushes effects on every call', () => {
-      function App(props) {
-        let [ctr, setCtr] = React.useState(0);
-        React.useEffect(() => {
-          props.callback(ctr);
-        });
-        return (
-          <button id="button" onClick={() => setCtr(x => x + 1)}>
-            {ctr}
-          </button>
-        );
-      }
-
-      let calledCounter = 0;
-      act(() => {
-        ReactDOM.render(
-          <App
-            callback={val => {
-              calledCounter = val;
-            }}
-          />,
-          container,
-        );
-      });
-      const button = document.getElementById('button');
-      function click() {
-        button.dispatchEvent(new MouseEvent('click', {bubbles: true}));
-      }
-
-      act(() => {
-        click();
-        click();
-        click();
-      });
-      expect(calledCounter).toBe(3);
-      act(click);
-      expect(calledCounter).toBe(4);
-      act(click);
-      expect(calledCounter).toBe(5);
-      expect(button.innerHTML).toBe('5');
+    afterEach(() => {
+      unmount(container);
+      document.body.removeChild(container);
     });
+    describe('sync', () => {
+      it('can use act to flush effects', () => {
+        function App() {
+          React.useEffect(() => {
+            Scheduler.yieldValue(100);
+          });
+          return null;
+        }
 
-    it('should flush effects recursively', () => {
-      function App() {
-        let [ctr, setCtr] = React.useState(0);
-        React.useEffect(() => {
-          if (ctr < 5) {
-            setCtr(x => x + 1);
-          }
+        act(() => {
+          render(<App />, container);
         });
-        return ctr;
-      }
 
-      act(() => {
-        ReactDOM.render(<App />, container);
+        expect(Scheduler).toHaveYielded([100]);
       });
 
-      expect(container.innerHTML).toBe('5');
-    });
-
-    it('detects setState being called outside of act(...)', () => {
-      let setValue = null;
-      function App() {
-        let [value, _setValue] = React.useState(0);
-        setValue = _setValue;
-        return (
-          <button id="button" onClick={() => setValue(2)}>
-            {value}
-          </button>
-        );
-      }
-      let button;
-      act(() => {
-        ReactDOM.render(<App />, container);
-        button = container.querySelector('#button');
-        button.dispatchEvent(new MouseEvent('click', {bubbles: true}));
-      });
-      expect(button.innerHTML).toBe('2');
-      expect(() => setValue(1)).toWarnDev([
-        'An update to App inside a test was not wrapped in act(...).',
-      ]);
-    });
-    describe('fake timers', () => {
-      beforeEach(() => {
-        jest.useFakeTimers();
-      });
-      afterEach(() => {
-        jest.useRealTimers();
-      });
-      it('lets a ticker update', () => {
+      it('flushes effects on every call', () => {
         function App() {
-          let [toggle, setToggle] = React.useState(0);
+          let [ctr, setCtr] = React.useState(0);
           React.useEffect(() => {
-            let timeout = setTimeout(() => {
-              setToggle(1);
-            }, 200);
-            return () => clearTimeout(timeout);
-          }, []);
-          return toggle;
+            Scheduler.yieldValue(ctr);
+          });
+          return (
+            <button id="button" onClick={() => setCtr(x => x + 1)}>
+              {ctr}
+            </button>
+          );
         }
 
         act(() => {
-          ReactDOM.render(<App />, container);
+          render(<App />, container);
         });
+        expect(Scheduler).toHaveYielded([0]);
+        const button = container.querySelector('#button');
+        function click() {
+          button.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+        }
+
         act(() => {
-          jest.runAllTimers();
+          click();
+          click();
+          click();
         });
-
-        expect(container.innerHTML).toBe('1');
+        // it consolidates the 3 updates, then fires the effect
+        expect(Scheduler).toHaveYielded([3]);
+        act(click);
+        expect(Scheduler).toHaveYielded([4]);
+        act(click);
+        expect(Scheduler).toHaveYielded([5]);
+        expect(button.innerHTML).toBe('5');
       });
-      it('can use the async version to catch microtasks', async () => {
+
+      it("should keep flushing effects until the're done", () => {
         function App() {
-          let [toggle, setToggle] = React.useState(0);
+          let [ctr, setCtr] = React.useState(0);
           React.useEffect(() => {
-            // just like the previous test, except we
-            // use a promise and schedule the update
-            // after it resolves
-            sleep(200).then(() => setToggle(1));
-          }, []);
-          return toggle;
+            if (ctr < 5) {
+              setCtr(x => x + 1);
+            }
+          });
+          return ctr;
         }
 
         act(() => {
-          ReactDOM.render(<App />, container);
-        });
-        await act(async () => {
-          jest.runAllTimers();
+          render(<App />, container);
         });
 
-        expect(container.innerHTML).toBe('1');
+        expect(container.innerHTML).toBe('5');
       });
-      it('can handle cascading promises with fake timers', async () => {
-        // this component triggers an effect, that waits a tick,
-        // then sets state. repeats this 5 times.
+
+      it('warns if a setState is called outside of act(...)', () => {
+        let setValue = null;
         function App() {
-          let [state, setState] = React.useState(0);
-          async function ticker() {
-            await null;
-            setState(x => x + 1);
-          }
-          React.useEffect(
-            () => {
-              ticker();
-            },
-            [Math.min(state, 4)],
-          );
-          return state;
+          let [value, _setValue] = React.useState(0);
+          setValue = _setValue;
+          return value;
         }
-        const el = document.createElement('div');
-        await act(async () => {
-          ReactDOM.render(<App />, el);
+
+        act(() => {
+          render(<App />, container);
         });
 
-        // all 5 ticks present and accounted for
-        expect(el.innerHTML).toBe('5');
+        expect(() => setValue(1)).toWarnDev([
+          'An update to App inside a test was not wrapped in act(...).',
+        ]);
       });
-      it('flushes immediate re-renders with act', () => {
-        function App() {
-          let [ctr, setCtr] = React.useState(0);
-          React.useEffect(() => {
-            if (ctr === 0) {
-              setCtr(1);
+      describe('fake timers', () => {
+        beforeEach(() => {
+          jest.useFakeTimers();
+        });
+        afterEach(() => {
+          jest.useRealTimers();
+        });
+        it('lets a ticker update', () => {
+          function App() {
+            let [toggle, setToggle] = React.useState(0);
+            React.useEffect(() => {
+              let timeout = setTimeout(() => {
+                setToggle(1);
+              }, 200);
+              return () => clearTimeout(timeout);
+            }, []);
+            return toggle;
+          }
+
+          act(() => {
+            render(<App />, container);
+          });
+          act(() => {
+            jest.runAllTimers();
+          });
+
+          expect(container.innerHTML).toBe('1');
+        });
+        it('can use the async version to catch microtasks', async () => {
+          function App() {
+            let [toggle, setToggle] = React.useState(0);
+            React.useEffect(() => {
+              // just like the previous test, except we
+              // use a promise and schedule the update
+              // after it resolves
+              sleep(200).then(() => setToggle(1));
+            }, []);
+            return toggle;
+          }
+
+          act(() => {
+            render(<App />, container);
+          });
+          await act(async () => {
+            jest.runAllTimers();
+          });
+
+          expect(container.innerHTML).toBe('1');
+        });
+        it('can handle cascading promises with fake timers', async () => {
+          // this component triggers an effect, that waits a tick,
+          // then sets state. repeats this 5 times.
+          function App() {
+            let [state, setState] = React.useState(0);
+            async function ticker() {
+              await null;
+              setState(x => x + 1);
             }
-            const timeout = setTimeout(() => setCtr(2), 1000);
-            return () => clearTimeout(timeout);
+            React.useEffect(
+              () => {
+                ticker();
+              },
+              [Math.min(state, 4)],
+            );
+            return state;
+          }
+
+          await act(async () => {
+            render(<App />, container);
           });
-          return ctr;
-        }
 
-        act(() => {
-          ReactDOM.render(<App />, container);
-          // Since the effects won't be flushed yet, this does not advance the timer
-          jest.runAllTimers();
+          // all 5 ticks present and accounted for
+          expect(container.innerHTML).toBe('5');
         });
+        it('flushes immediate re-renders with act', () => {
+          function App() {
+            let [ctr, setCtr] = React.useState(0);
+            React.useEffect(() => {
+              if (ctr === 0) {
+                setCtr(1);
+              }
+              const timeout = setTimeout(() => setCtr(2), 1000);
+              return () => clearTimeout(timeout);
+            });
+            return ctr;
+          }
 
-        expect(container.innerHTML).toBe('1');
+          act(() => {
+            render(<App />, container);
+            // Since effects haven't been flushed yet, this does not advance the timer
+            jest.runAllTimers();
+          });
 
-        act(() => {
-          jest.runAllTimers();
+          expect(container.innerHTML).toBe('1');
+
+          act(() => {
+            jest.runAllTimers();
+          });
+
+          expect(container.innerHTML).toBe('2');
         });
+      });
 
-        expect(container.innerHTML).toBe('2');
+      it('warns if you return a value inside act', () => {
+        expect(() => act(() => null)).toWarnDev(
+          [
+            'The callback passed to act(...) function must return undefined, or a Promise.',
+          ],
+          {withoutStack: true},
+        );
+        expect(() => act(() => 123)).toWarnDev(
+          [
+            'The callback passed to act(...) function must return undefined, or a Promise.',
+          ],
+          {withoutStack: true},
+        );
       });
-    });
 
-    it('warns if you return a value inside act', () => {
-      expect(() => act(() => null)).toWarnDev(
-        [
-          'The callback passed to act(...) function must return undefined, or a Promise.',
-        ],
-        {withoutStack: true},
-      );
-      expect(() => act(() => 123)).toWarnDev(
-        [
-          'The callback passed to act(...) function must return undefined, or a Promise.',
-        ],
-        {withoutStack: true},
-      );
+      it('warns if you try to await a sync .act call', () => {
+        expect(() => act(() => {}).then(() => {})).toWarnDev(
+          [
+            'Do not await the result of calling act(...) with sync logic, it is not a Promise.',
+          ],
+          {withoutStack: true},
+        );
+      });
     });
+    describe('asynchronous tests', () => {
+      it('can handle timers', async () => {
+        function App() {
+          let [ctr, setCtr] = React.useState(0);
+          function doSomething() {
+            setTimeout(() => {
+              setCtr(1);
+            }, 50);
+          }
 
-    it('warns if you try to await an .act call', () => {
-      expect(() => act(() => {}).then(() => {})).toWarnDev(
-        [
-          'Do not await the result of calling act(...) with sync logic, it is not a Promise.',
-        ],
-        {withoutStack: true},
-      );
-    });
-  });
-  describe('asynchronous tests', () => {
-    it('can handle timers', async () => {
-      function App() {
-        let [ctr, setCtr] = React.useState(0);
-        function doSomething() {
-          setTimeout(() => {
-            setCtr(1);
-          }, 50);
+          React.useEffect(() => {
+            doSomething();
+          }, []);
+          return ctr;
         }
-
-        React.useEffect(() => {
-          doSomething();
-        }, []);
-        return ctr;
-      }
-      const el = document.createElement('div');
-      await act(async () => {
         act(() => {
-          ReactDOM.render(<App />, el);
+          render(<App />, container);
         });
-
-        await sleep(100);
-        expect(el.innerHTML).toBe('1');
+        await act(async () => {
+          await sleep(100);
+        });
+        expect(container.innerHTML).toBe('1');
       });
-    });
 
-    it('can handle async/await', async () => {
-      function App() {
-        let [ctr, setCtr] = React.useState(0);
-        async function someAsyncFunction() {
-          // queue a bunch of promises to be sure they all flush
-          await null;
-          await null;
-          await null;
-          setCtr(1);
+      it('can handle async/await', async () => {
+        function App() {
+          let [ctr, setCtr] = React.useState(0);
+          async function someAsyncFunction() {
+            // queue a bunch of promises to be sure they all flush
+            await null;
+            await null;
+            await null;
+            setCtr(1);
+          }
+          React.useEffect(() => {
+            someAsyncFunction();
+          }, []);
+          return ctr;
         }
-        React.useEffect(() => {
-          someAsyncFunction();
-        }, []);
-        return ctr;
-      }
-      const el = document.createElement('div');
 
-      await act(async () => {
-        act(() => {
-          ReactDOM.render(<App />, el);
+        await act(async () => {
+          act(() => {
+            render(<App />, container);
+          });
+          // pending promises will close before this ends
         });
-        // pending promises will close before this ends
+        expect(container.innerHTML).toEqual('1');
       });
-      expect(el.innerHTML).toEqual('1');
-    });
 
-    it('warns if you do not await an act call', async () => {
-      spyOnDevAndProd(console, 'error');
-      act(async () => {});
-      // it's annoying that we have to wait a tick before this warning comes in
-      await sleep(0);
-      if (__DEV__) {
-        expect(console.error.calls.count()).toEqual(1);
-        expect(console.error.calls.argsFor(0)[0]).toMatch(
-          'You called act(async () => ...) without await.',
-        );
-      }
-    });
+      it('warns if you do not await an act call', async () => {
+        spyOnDevAndProd(console, 'error');
+        act(async () => {});
+        // it's annoying that we have to wait a tick before this warning comes in
+        await sleep(0);
+        if (__DEV__) {
+          expect(console.error.calls.count()).toEqual(1);
+          expect(console.error.calls.argsFor(0)[0]).toMatch(
+            'You called act(async () => ...) without await.',
+          );
+        }
+      });
+
+      it('warns if you try to interleave multiple act calls', async () => {
+        spyOnDevAndProd(console, 'error');
+        // let's try to cheat and spin off a 'thread' with an act call
+        (async () => {
+          await act(async () => {
+            await sleep(50);
+          });
+        })();
 
-    it('warns if you try to interleave multiple act calls', async () => {
-      spyOnDevAndProd(console, 'error');
-      // let's try to cheat and spin off a 'thread' with an act call
-      (async () => {
         await act(async () => {
-          await sleep(50);
+          await sleep(100);
         });
-      })();
 
-      await act(async () => {
-        await sleep(100);
+        await sleep(150);
+        if (__DEV__) {
+          expect(console.error).toHaveBeenCalledTimes(1);
+        }
       });
 
-      await sleep(150);
-      if (__DEV__) {
-        expect(console.error).toHaveBeenCalledTimes(1);
-      }
-    });
-
-    it('commits and effects are guaranteed to be flushed', async () => {
-      function App(props) {
-        let [state, setState] = React.useState(0);
-        async function something() {
-          await null;
-          setState(1);
+      it('commits and effects are guaranteed to be flushed', async () => {
+        function App() {
+          let [state, setState] = React.useState(0);
+          async function something() {
+            await null;
+            setState(1);
+          }
+          React.useEffect(() => {
+            something();
+          }, []);
+          React.useEffect(() => {
+            Scheduler.yieldValue(state);
+          });
+          return state;
         }
-        React.useEffect(() => {
-          something();
-        }, []);
-        React.useEffect(() => {
-          props.callback();
-        });
-        return state;
-      }
-      let ctr = 0;
-      const div = document.createElement('div');
 
-      await act(async () => {
-        act(() => {
-          ReactDOM.render(<App callback={() => ctr++} />, div);
+        await act(async () => {
+          act(() => {
+            render(<App />, container);
+          });
+          expect(container.innerHTML).toBe('0');
+          expect(Scheduler).toHaveYielded([0]);
         });
-        expect(div.innerHTML).toBe('0');
-        expect(ctr).toBe(1);
+        // this may seem odd, but it matches user behaviour -
+        // a flash of "0" followed by "1"
+
+        expect(container.innerHTML).toBe('1');
+        expect(Scheduler).toHaveYielded([1]);
       });
-      // this may seem odd, but it matches user behaviour -
-      // a flash of "0" followed by "1"
 
-      expect(div.innerHTML).toBe('1');
-      expect(ctr).toBe(2);
-    });
+      it('propagates errors', async () => {
+        let err;
+        try {
+          await act(async () => {
+            await sleep(100);
+            throw new Error('some error');
+          });
+        } catch (_err) {
+          err = _err;
+        } finally {
+          expect(err instanceof Error).toBe(true);
+          expect(err.message).toBe('some error');
+        }
+      });
+      it('can handle cascading promises', async () => {
+        // this component triggers an effect, that waits a tick,
+        // then sets state. repeats this 5 times.
+        function App() {
+          let [state, setState] = React.useState(0);
+          async function ticker() {
+            await null;
+            setState(x => x + 1);
+          }
+          React.useEffect(
+            () => {
+              Scheduler.yieldValue(state);
+              ticker();
+            },
+            [Math.min(state, 4)],
+          );
+          return state;
+        }
 
-    it('propagates errors', async () => {
-      let err;
-      try {
         await act(async () => {
-          throw new Error('some error');
+          render(<App />, container);
         });
-      } catch (_err) {
-        err = _err;
-      } finally {
-        expect(err instanceof Error).toBe(true);
-        expect(err.message).toBe('some error');
-      }
-    });
-    it('can handle cascading promises', async () => {
-      // this component triggers an effect, that waits a tick,
-      // then sets state. repeats this 5 times.
-      function App() {
-        let [state, setState] = React.useState(0);
-        async function ticker() {
-          await null;
-          setState(x => x + 1);
-        }
-        React.useEffect(
-          () => {
-            ticker();
-          },
-          [Math.min(state, 4)],
-        );
-        return state;
-      }
-      const el = document.createElement('div');
-      await act(async () => {
-        ReactDOM.render(<App />, el);
+        // all 5 ticks present and accounted for
+        expect(Scheduler).toHaveYielded([0, 1, 2, 3, 4]);
+        expect(container.innerHTML).toBe('5');
       });
-      // all 5 ticks present and accounted for
-      expect(el.innerHTML).toBe('5');
     });
-  });
 
-  describe('interaction tracing', () => {
-    if (__DEV__) {
-      it('should correctly trace interactions for sync roots', () => {
-        let expectedInteraction;
+    describe('interaction tracing', () => {
+      if (__DEV__) {
+        it('should correctly trace interactions for sync roots', () => {
+          let expectedInteraction;
+
+          const Component = jest.fn(() => {
+            expect(expectedInteraction).toBeDefined();
 
-        const Component = jest.fn(() => {
-          expect(expectedInteraction).toBeDefined();
+            const interactions = SchedulerTracing.unstable_getCurrent();
+            expect(interactions.size).toBe(1);
+            expect(interactions).toContain(expectedInteraction);
 
-          const interactions = SchedulerTracing.unstable_getCurrent();
-          expect(interactions.size).toBe(1);
-          expect(interactions).toContain(expectedInteraction);
+            return null;
+          });
 
-          return null;
-        });
+          act(() => {
+            SchedulerTracing.unstable_trace(
+              'mount traced inside act',
+              performance.now(),
+              () => {
+                const interactions = SchedulerTracing.unstable_getCurrent();
+                expect(interactions.size).toBe(1);
+                expectedInteraction = Array.from(interactions)[0];
+
+                render(<Component />, container);
+              },
+            );
+          });
+
+          act(() => {
+            SchedulerTracing.unstable_trace(
+              'update traced inside act',
+              performance.now(),
+              () => {
+                const interactions = SchedulerTracing.unstable_getCurrent();
+                expect(interactions.size).toBe(1);
+                expectedInteraction = Array.from(interactions)[0];
+
+                render(<Component />, container);
+              },
+            );
+          });
+
+          const secondContainer = document.createElement('div');
 
-        act(() => {
           SchedulerTracing.unstable_trace(
-            'mount traced inside act',
+            'mount traced outside act',
             performance.now(),
             () => {
-              const interactions = SchedulerTracing.unstable_getCurrent();
-              expect(interactions.size).toBe(1);
-              expectedInteraction = Array.from(interactions)[0];
+              act(() => {
+                const interactions = SchedulerTracing.unstable_getCurrent();
+                expect(interactions.size).toBe(1);
+                expectedInteraction = Array.from(interactions)[0];
 
-              ReactDOM.render(<Component />, container);
+                render(<Component />, secondContainer);
+              });
             },
           );
-        });
 
-        act(() => {
           SchedulerTracing.unstable_trace(
-            'update traced inside act',
+            'update traced outside act',
             performance.now(),
             () => {
-              const interactions = SchedulerTracing.unstable_getCurrent();
-              expect(interactions.size).toBe(1);
-              expectedInteraction = Array.from(interactions)[0];
+              act(() => {
+                const interactions = SchedulerTracing.unstable_getCurrent();
+                expect(interactions.size).toBe(1);
+                expectedInteraction = Array.from(interactions)[0];
 
-              ReactDOM.render(<Component />, container);
+                render(<Component />, secondContainer);
+              });
             },
           );
-        });
-
-        const secondContainer = document.createElement('div');
-
-        SchedulerTracing.unstable_trace(
-          'mount traced outside act',
-          performance.now(),
-          () => {
-            act(() => {
-              const interactions = SchedulerTracing.unstable_getCurrent();
-              expect(interactions.size).toBe(1);
-              expectedInteraction = Array.from(interactions)[0];
 
-              ReactDOM.render(<Component />, secondContainer);
-            });
-          },
-        );
-
-        SchedulerTracing.unstable_trace(
-          'update traced outside act',
-          performance.now(),
-          () => {
-            act(() => {
-              const interactions = SchedulerTracing.unstable_getCurrent();
-              expect(interactions.size).toBe(1);
-              expectedInteraction = Array.from(interactions)[0];
-
-              ReactDOM.render(<Component />, secondContainer);
-            });
-          },
-        );
-
-        expect(Component).toHaveBeenCalledTimes(4);
-      });
-    }
+          expect(Component).toHaveBeenCalledTimes(4);
+          unmount(secondContainer);
+        });
+      }
+    });
   });
-});
+}
diff --git a/packages/react-dom/src/test-utils/ReactTestUtilsAct.js b/packages/react-dom/src/test-utils/ReactTestUtilsAct.js
index 99cb73ede7c10..9667c1fbd5bbe 100644
--- a/packages/react-dom/src/test-utils/ReactTestUtilsAct.js
+++ b/packages/react-dom/src/test-utils/ReactTestUtilsAct.js
@@ -12,7 +12,9 @@ import type {Thenable} from 'react-reconciler/src/ReactFiberScheduler';
 import warningWithoutStack from 'shared/warningWithoutStack';
 import ReactDOM from 'react-dom';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
+import {warnAboutMissingMockScheduler} from 'shared/ReactFeatureFlags';
 import enqueueTask from 'shared/enqueueTask';
+import * as Scheduler from 'scheduler';
 
 // Keep in sync with ReactDOMUnstableNativeDependencies.js
 // ReactDOM.js, and ReactTestUtils.js:
@@ -40,16 +42,33 @@ const {ReactShouldWarnActingUpdates} = ReactSharedInternals;
 // this implementation should be exactly the same in
 // ReactTestUtilsAct.js, ReactTestRendererAct.js, createReactNoop.js
 
-// we track the 'depth' of the act() calls with this counter,
-// so we can tell if any async act() calls try to run in parallel.
-let actingUpdatesScopeDepth = 0;
+let hasWarnedAboutMissingMockScheduler = false;
+const flushWork =
+  Scheduler.unstable_flushWithoutYielding ||
+  function() {
+    if (warnAboutMissingMockScheduler === true) {
+      if (hasWarnedAboutMissingMockScheduler === false) {
+        warningWithoutStack(
+          null,
+          'Starting from React v17, the "scheduler" module will need to be mocked ' +
+            'to guarantee consistent behaviour across tests and browsers. To fix this, add the following ' +
+            "to the top of your tests, or in your framework's global config file -\n\n" +
+            'As an example, for jest - \n' +
+            "jest.mock('scheduler', () => require.requireActual('scheduler/unstable_mock'));\n\n" +
+            'For more info, visit https://fb.me/react-mock-scheduler',
+        );
+        hasWarnedAboutMissingMockScheduler = true;
+      }
+    }
+    while (flushPassiveEffects()) {}
+  };
 
-function flushEffectsAndMicroTasks(onDone: (err: ?Error) => void) {
+function flushWorkAndMicroTasks(onDone: (err: ?Error) => void) {
   try {
-    flushPassiveEffects();
+    flushWork();
     enqueueTask(() => {
-      if (flushPassiveEffects()) {
-        flushEffectsAndMicroTasks(onDone);
+      if (flushWork()) {
+        flushWorkAndMicroTasks(onDone);
       } else {
         onDone();
       }
@@ -59,6 +78,11 @@ function flushEffectsAndMicroTasks(onDone: (err: ?Error) => void) {
   }
 }
 
+// we track the 'depth' of the act() calls with this counter,
+// so we can tell if any async act() calls try to run in parallel.
+
+let actingUpdatesScopeDepth = 0;
+
 function act(callback: () => Thenable) {
   let previousActingUpdatesScopeDepth;
   if (__DEV__) {
@@ -119,7 +143,7 @@ function act(callback: () => Thenable) {
         called = true;
         result.then(
           () => {
-            flushEffectsAndMicroTasks((err: ?Error) => {
+            flushWorkAndMicroTasks((err: ?Error) => {
               onDone();
               if (err) {
                 reject(err);
@@ -147,7 +171,7 @@ function act(callback: () => Thenable) {
 
     // flush effects until none remain, and cleanup
     try {
-      while (flushPassiveEffects()) {}
+      flushWork();
       onDone();
     } catch (err) {
       onDone();
diff --git a/packages/react-noop-renderer/src/createReactNoop.js b/packages/react-noop-renderer/src/createReactNoop.js
index 4cdb0fe4beb99..e7bd09890a7bc 100644
--- a/packages/react-noop-renderer/src/createReactNoop.js
+++ b/packages/react-noop-renderer/src/createReactNoop.js
@@ -32,7 +32,10 @@ import warning from 'shared/warning';
 import enqueueTask from 'shared/enqueueTask';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import warningWithoutStack from 'shared/warningWithoutStack';
-import {enableEventAPI} from 'shared/ReactFeatureFlags';
+import {
+  warnAboutMissingMockScheduler,
+  enableEventAPI,
+} from 'shared/ReactFeatureFlags';
 import {ConcurrentRoot, BatchedRoot, LegacyRoot} from 'shared/ReactRootTags';
 
 type EventTargetChildElement = {
@@ -652,14 +655,33 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
   // this act() implementation should be exactly the same in
   // ReactTestUtilsAct.js, ReactTestRendererAct.js, createReactNoop.js
 
-  let actingUpdatesScopeDepth = 0;
+  let hasWarnedAboutMissingMockScheduler = false;
+  const flushWork =
+    Scheduler.unstable_flushWithoutYielding ||
+    function() {
+      if (warnAboutMissingMockScheduler === true) {
+        if (hasWarnedAboutMissingMockScheduler === false) {
+          warningWithoutStack(
+            null,
+            'Starting from React v17, the "scheduler" module will need to be mocked ' +
+              'to guarantee consistent behaviour across tests and browsers. To fix this, add the following ' +
+              "to the top of your tests, or in your framework's global config file -\n\n" +
+              'As an example, for jest - \n' +
+              "jest.mock('scheduler', () => require.requireActual('scheduler/unstable_mock'));\n\n" +
+              'For more info, visit https://fb.me/react-mock-scheduler',
+          );
+          hasWarnedAboutMissingMockScheduler = true;
+        }
+      }
+      while (flushPassiveEffects()) {}
+    };
 
-  function flushEffectsAndMicroTasks(onDone: (err: ?Error) => void) {
+  function flushWorkAndMicroTasks(onDone: (err: ?Error) => void) {
     try {
-      flushPassiveEffects();
+      flushWork();
       enqueueTask(() => {
-        if (flushPassiveEffects()) {
-          flushEffectsAndMicroTasks(onDone);
+        if (flushWork()) {
+          flushWorkAndMicroTasks(onDone);
         } else {
           onDone();
         }
@@ -669,6 +691,11 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
     }
   }
 
+  // we track the 'depth' of the act() calls with this counter,
+  // so we can tell if any async act() calls try to run in parallel.
+
+  let actingUpdatesScopeDepth = 0;
+
   function act(callback: () => Thenable) {
     let previousActingUpdatesScopeDepth;
     if (__DEV__) {
@@ -729,7 +756,7 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
           called = true;
           result.then(
             () => {
-              flushEffectsAndMicroTasks((err: ?Error) => {
+              flushWorkAndMicroTasks((err: ?Error) => {
                 onDone();
                 if (err) {
                   reject(err);
@@ -757,7 +784,7 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
 
       // flush effects until none remain, and cleanup
       try {
-        while (flushPassiveEffects()) {}
+        flushWork();
         onDone();
       } catch (err) {
         onDone();
diff --git a/packages/react-reconciler/src/__tests__/ReactBatchedMode-test.internal.js b/packages/react-reconciler/src/__tests__/ReactBatchedMode-test.internal.js
index 670b0e49fb1bc..ea33035ca37a2 100644
--- a/packages/react-reconciler/src/__tests__/ReactBatchedMode-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactBatchedMode-test.internal.js
@@ -1,7 +1,6 @@
 let React;
 let ReactFeatureFlags;
 let ReactNoop;
-let act;
 let Scheduler;
 let ReactCache;
 let Suspense;
@@ -15,7 +14,6 @@ describe('ReactBatchedMode', () => {
     ReactFeatureFlags.replayFailedUnitOfWorkWithInvokeGuardedCallback = false;
     React = require('react');
     ReactNoop = require('react-noop-renderer');
-    act = ReactNoop.act;
     Scheduler = require('scheduler');
     ReactCache = require('react-cache');
     Suspense = React.Suspense;
@@ -146,10 +144,10 @@ describe('ReactBatchedMode', () => {
     expect(root).toMatchRenderedOutput('A0B0');
 
     // Schedule a batched update to the first sibling
-    act(() => foo1.current.setStep(1));
+    ReactNoop.batchedUpdates(() => foo1.current.setStep(1));
 
     // Before it flushes, update the second sibling inside flushSync
-    act(() =>
+    ReactNoop.batchedUpdates(() =>
       ReactNoop.flushSync(() => {
         foo2.current.setStep(1);
       }),
diff --git a/packages/react-reconciler/src/__tests__/ReactHooks-test.internal.js b/packages/react-reconciler/src/__tests__/ReactHooks-test.internal.js
index 50276bd8b7e06..447103d85e2da 100644
--- a/packages/react-reconciler/src/__tests__/ReactHooks-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactHooks-test.internal.js
@@ -95,7 +95,7 @@ describe('ReactHooks', () => {
       setCounter2(1);
     });
 
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Parent: 1, 1',
       'Child: 1, 1',
       'Effect: 1, 1',
@@ -103,7 +103,7 @@ describe('ReactHooks', () => {
 
     // Update that bails out.
     act(() => setCounter1(1));
-    expect(Scheduler).toFlushAndYield(['Parent: 1, 1']);
+    expect(Scheduler).toHaveYielded(['Parent: 1, 1']);
 
     // This time, one of the state updates but the other one doesn't. So we
     // can't bail out.
@@ -112,7 +112,7 @@ describe('ReactHooks', () => {
       setCounter2(2);
     });
 
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Parent: 1, 2',
       'Child: 1, 2',
       'Effect: 1, 2',
@@ -130,14 +130,15 @@ describe('ReactHooks', () => {
 
     // Because the final values are the same as the current values, the
     // component bails out.
-    expect(Scheduler).toFlushAndYield(['Parent: 1, 2']);
+    expect(Scheduler).toHaveYielded(['Parent: 1, 2']);
 
     // prepare to check SameValue
     act(() => {
       setCounter1(0 / -1);
       setCounter2(NaN);
     });
-    expect(Scheduler).toFlushAndYield([
+
+    expect(Scheduler).toHaveYielded([
       'Parent: 0, NaN',
       'Child: 0, NaN',
       'Effect: 0, NaN',
@@ -151,13 +152,13 @@ describe('ReactHooks', () => {
       setCounter2(NaN);
     });
 
-    expect(Scheduler).toFlushAndYield(['Parent: 0, NaN']);
+    expect(Scheduler).toHaveYielded(['Parent: 0, NaN']);
 
     // check if changing negative 0 to positive 0 does not bail out
     act(() => {
       setCounter1(0);
     });
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Parent: 0, NaN',
       'Child: 0, NaN',
       'Effect: 0, NaN',
@@ -201,14 +202,14 @@ describe('ReactHooks', () => {
       setCounter2(1);
     });
 
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Parent: 1, 1 (light)',
       'Child: 1, 1 (light)',
     ]);
 
     // Update that bails out.
     act(() => setCounter1(1));
-    expect(Scheduler).toFlushAndYield(['Parent: 1, 1 (light)']);
+    expect(Scheduler).toHaveYielded(['Parent: 1, 1 (light)']);
 
     // This time, one of the state updates but the other one doesn't. So we
     // can't bail out.
@@ -217,7 +218,7 @@ describe('ReactHooks', () => {
       setCounter2(2);
     });
 
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Parent: 1, 2 (light)',
       'Child: 1, 2 (light)',
     ]);
@@ -227,10 +228,10 @@ describe('ReactHooks', () => {
     act(() => {
       setCounter1(1);
       setCounter2(2);
+      root.update(<Parent theme="dark" />);
     });
 
-    root.update(<Parent theme="dark" />);
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Parent: 1, 2 (dark)',
       'Child: 1, 2 (dark)',
     ]);
@@ -239,10 +240,10 @@ describe('ReactHooks', () => {
     act(() => {
       setCounter1(1);
       setCounter2(2);
+      root.update(<Parent theme="dark" />);
     });
 
-    root.update(<Parent theme="dark" />);
-    expect(Scheduler).toFlushAndYield(['Parent: 1, 2 (dark)']);
+    expect(Scheduler).toHaveYielded(['Parent: 1, 2 (dark)']);
   });
 
   it('warns about setState second argument', () => {
@@ -275,7 +276,7 @@ describe('ReactHooks', () => {
         'declare it in the component body with useEffect().',
       {withoutStack: true},
     );
-    expect(Scheduler).toFlushAndYield(['Count: 1']);
+    expect(Scheduler).toHaveYielded(['Count: 1']);
     expect(root).toMatchRenderedOutput('1');
   });
 
@@ -309,7 +310,7 @@ describe('ReactHooks', () => {
         'declare it in the component body with useEffect().',
       {withoutStack: true},
     );
-    expect(Scheduler).toFlushAndYield(['Count: 1']);
+    expect(Scheduler).toHaveYielded(['Count: 1']);
     expect(root).toMatchRenderedOutput('1');
   });
 
@@ -347,14 +348,16 @@ describe('ReactHooks', () => {
       });
       return <Child text={text} />;
     }
-
     const root = ReactTestRenderer.create(null, {unstable_isConcurrent: true});
-    root.update(
-      <ThemeProvider>
-        <Parent />
-      </ThemeProvider>,
-    );
-    expect(Scheduler).toFlushAndYield([
+    act(() => {
+      root.update(
+        <ThemeProvider>
+          <Parent />
+        </ThemeProvider>,
+      );
+    });
+
+    expect(Scheduler).toHaveYielded([
       'Theme: light',
       'Parent: 0 (light)',
       'Child: 0 (light)',
@@ -370,7 +373,7 @@ describe('ReactHooks', () => {
 
     // Normal update
     act(() => setCounter(1));
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Parent: 1 (light)',
       'Child: 1 (light)',
       'Effect: 1 (light)',
@@ -379,7 +382,7 @@ describe('ReactHooks', () => {
 
     // Update that doesn't change state, so it bails out
     act(() => setCounter(1));
-    expect(Scheduler).toFlushAndYield(['Parent: 1 (light)']);
+    expect(Scheduler).toHaveYielded(['Parent: 1 (light)']);
     expect(root).toMatchRenderedOutput('1 (light)');
 
     // Update that doesn't change state, but the context changes, too, so it
@@ -389,7 +392,7 @@ describe('ReactHooks', () => {
       setTheme('dark');
     });
 
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Theme: dark',
       'Parent: 1 (dark)',
       'Child: 1 (dark)',
@@ -424,7 +427,7 @@ describe('ReactHooks', () => {
 
     // Normal update
     act(() => setCounter(1));
-    expect(Scheduler).toFlushAndYield(['Parent: 1', 'Child: 1', 'Effect: 1']);
+    expect(Scheduler).toHaveYielded(['Parent: 1', 'Child: 1', 'Effect: 1']);
     expect(root).toMatchRenderedOutput('1');
 
     // Update to the same state. React doesn't know if the queue is empty
@@ -432,7 +435,7 @@ describe('ReactHooks', () => {
     // enter the render phase before we can bail out. But we bail out before
     // rendering the child, and we don't fire any effects.
     act(() => setCounter(1));
-    expect(Scheduler).toFlushAndYield(['Parent: 1']);
+    expect(Scheduler).toHaveYielded(['Parent: 1']);
     expect(root).toMatchRenderedOutput('1');
 
     // Update to the same state again. This times, neither fiber has pending
@@ -443,14 +446,14 @@ describe('ReactHooks', () => {
 
     // This changes the state to something different so it renders normally.
     act(() => setCounter(2));
-    expect(Scheduler).toFlushAndYield(['Parent: 2', 'Child: 2', 'Effect: 2']);
+    expect(Scheduler).toHaveYielded(['Parent: 2', 'Child: 2', 'Effect: 2']);
     expect(root).toMatchRenderedOutput('2');
 
     // prepare to check SameValue
     act(() => {
       setCounter(0);
     });
-    expect(Scheduler).toFlushAndYield(['Parent: 0', 'Child: 0', 'Effect: 0']);
+    expect(Scheduler).toHaveYielded(['Parent: 0', 'Child: 0', 'Effect: 0']);
     expect(root).toMatchRenderedOutput('0');
 
     // Update to the same state for the first time to flush the queue
@@ -458,7 +461,7 @@ describe('ReactHooks', () => {
       setCounter(0);
     });
 
-    expect(Scheduler).toFlushAndYield(['Parent: 0']);
+    expect(Scheduler).toHaveYielded(['Parent: 0']);
     expect(root).toMatchRenderedOutput('0');
 
     // Update again to the same state. Should bail out.
@@ -472,7 +475,7 @@ describe('ReactHooks', () => {
     act(() => {
       setCounter(0 / -1);
     });
-    expect(Scheduler).toFlushAndYield(['Parent: 0', 'Child: 0', 'Effect: 0']);
+    expect(Scheduler).toHaveYielded(['Parent: 0', 'Child: 0', 'Effect: 0']);
     expect(root).toMatchRenderedOutput('0');
   });
 
@@ -503,7 +506,7 @@ describe('ReactHooks', () => {
         return value;
       });
     };
-    act(() => {
+    ReactTestRenderer.unstable_batchedUpdates(() => {
       update(0);
       update(0);
       update(0);
@@ -564,7 +567,7 @@ describe('ReactHooks', () => {
     };
 
     // Update at normal priority
-    act(() => update(n => n * 100));
+    ReactTestRenderer.unstable_batchedUpdates(() => update(n => n * 100));
 
     // The new state is eagerly computed.
     expect(Scheduler).toHaveYielded(['Compute state (1 -> 100)']);
diff --git a/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.internal.js b/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.internal.js
index 3a2bf76122b10..63081362209ac 100644
--- a/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactHooksWithNoopRenderer-test.internal.js
@@ -80,7 +80,7 @@ describe('ReactHooksWithNoopRenderer', () => {
     expect(ReactNoop.getChildren()).toEqual([span('Count: 0')]);
 
     // Schedule some updates
-    act(() => {
+    ReactNoop.batchedUpdates(() => {
       counter.current.updateCount(1);
       counter.current.updateCount(count => count + 10);
     });
@@ -189,11 +189,11 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(ReactNoop.getChildren()).toEqual([span('Count: 0')]);
 
       act(() => counter.current.updateCount(1));
-      expect(Scheduler).toFlushAndYield(['Count: 1']);
+      expect(Scheduler).toHaveYielded(['Count: 1']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 1')]);
 
       act(() => counter.current.updateCount(count => count + 10));
-      expect(Scheduler).toFlushAndYield(['Count: 11']);
+      expect(Scheduler).toHaveYielded(['Count: 11']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 11')]);
     });
 
@@ -213,7 +213,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(ReactNoop.getChildren()).toEqual([span('Count: 42')]);
 
       act(() => counter.current.updateCount(7));
-      expect(Scheduler).toFlushAndYield(['Count: 7']);
+      expect(Scheduler).toHaveYielded(['Count: 7']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 7')]);
     });
 
@@ -231,10 +231,10 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(ReactNoop.getChildren()).toEqual([span('Count: 0')]);
 
       act(() => counter.current.updateCount(7));
-      expect(Scheduler).toFlushAndYield(['Count: 7']);
+      expect(Scheduler).toHaveYielded(['Count: 7']);
 
       act(() => counter.current.updateLabel('Total'));
-      expect(Scheduler).toFlushAndYield(['Total: 7']);
+      expect(Scheduler).toHaveYielded(['Total: 7']);
     });
 
     it('returns the same updater function every time', () => {
@@ -249,11 +249,11 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(ReactNoop.getChildren()).toEqual([span('Count: 0')]);
 
       act(() => updaters[0](1));
-      expect(Scheduler).toFlushAndYield(['Count: 1']);
+      expect(Scheduler).toHaveYielded(['Count: 1']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 1')]);
 
       act(() => updaters[0](count => count + 10));
-      expect(Scheduler).toFlushAndYield(['Count: 11']);
+      expect(Scheduler).toHaveYielded(['Count: 11']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 11')]);
 
       expect(updaters).toEqual([updaters[0], updaters[0], updaters[0]]);
@@ -298,7 +298,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(ReactNoop.getChildren()).toEqual([span('Count: 0')]);
 
       act(() => _updateCount(1));
-      expect(Scheduler).toFlushAndYield(['Count: 1']);
+      expect(Scheduler).toHaveYielded(['Count: 1']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 1')]);
     });
   });
@@ -484,7 +484,7 @@ describe('ReactHooksWithNoopRenderer', () => {
         counter.current.dispatch('reset');
       });
       ReactNoop.render(<Counter ref={counter} />);
-      expect(Scheduler).toFlushAndYield([
+      expect(Scheduler).toHaveYielded([
         'Render: 0',
         'Render: 1',
         'Render: 11',
@@ -524,7 +524,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(ReactNoop.getChildren()).toEqual([span('Count: 0')]);
 
       act(() => counter.current.dispatch(INCREMENT));
-      expect(Scheduler).toFlushAndYield(['Count: 1']);
+      expect(Scheduler).toHaveYielded(['Count: 1']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 1')]);
       act(() => {
         counter.current.dispatch(DECREMENT);
@@ -532,7 +532,7 @@ describe('ReactHooksWithNoopRenderer', () => {
         counter.current.dispatch(DECREMENT);
       });
 
-      expect(Scheduler).toFlushAndYield(['Count: -2']);
+      expect(Scheduler).toHaveYielded(['Count: -2']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: -2')]);
     });
 
@@ -566,7 +566,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(ReactNoop.getChildren()).toEqual([span('Count: 10')]);
 
       act(() => counter.current.dispatch(INCREMENT));
-      expect(Scheduler).toFlushAndYield(['Count: 11']);
+      expect(Scheduler).toHaveYielded(['Count: 11']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 11')]);
 
       act(() => {
@@ -575,7 +575,7 @@ describe('ReactHooksWithNoopRenderer', () => {
         counter.current.dispatch(DECREMENT);
       });
 
-      expect(Scheduler).toFlushAndYield(['Count: 8']);
+      expect(Scheduler).toHaveYielded(['Count: 8']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 8')]);
     });
 
@@ -600,7 +600,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       expect(Scheduler).toFlushAndYield(['Count: 0']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 0')]);
 
-      act(() => {
+      ReactNoop.batchedUpdates(() => {
         counter.current.dispatch(INCREMENT);
         counter.current.dispatch(INCREMENT);
         counter.current.dispatch(INCREMENT);
@@ -884,8 +884,12 @@ describe('ReactHooksWithNoopRenderer', () => {
 
       // A discrete event forces the passive effect to be flushed --
       // updateCount(1) happens first, so 2 wins.
+
       ReactNoop.interactiveUpdates(() => {
-        act(() => _updateCount(2));
+        // (use batchedUpdates to silence the act() warning)
+        ReactNoop.batchedUpdates(() => {
+          _updateCount(2);
+        });
       });
       expect(Scheduler).toHaveYielded(['Will set count to 1']);
       expect(Scheduler).toFlushAndYield(['Count: 2']);
@@ -936,7 +940,8 @@ describe('ReactHooksWithNoopRenderer', () => {
       // A discrete event forces the passive effect to be flushed --
       // updateCount(1) happens first, so 2 wins.
       ReactNoop.interactiveUpdates(() => {
-        act(() => _updateCount(2));
+        // use batchedUpdates to silence the act warning
+        ReactNoop.batchedUpdates(() => _updateCount(2));
       });
       expect(Scheduler).toHaveYielded(['Will set count to 1']);
       expect(Scheduler).toFlushAndYield(['Count: 2']);
@@ -1527,7 +1532,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       ]);
 
       act(button.current.increment);
-      expect(Scheduler).toFlushAndYield([
+      expect(Scheduler).toHaveYielded([
         // Button should not re-render, because its props haven't changed
         // 'Increment',
         'Count: 1',
@@ -1551,7 +1556,7 @@ describe('ReactHooksWithNoopRenderer', () => {
 
       // Callback should have updated
       act(button.current.increment);
-      expect(Scheduler).toFlushAndYield(['Count: 11']);
+      expect(Scheduler).toHaveYielded(['Count: 11']);
       expect(ReactNoop.getChildren()).toEqual([
         span('Increment'),
         span('Count: 11'),
@@ -1754,7 +1759,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       act(() => {
         counter.current.dispatch(INCREMENT);
       });
-      expect(Scheduler).toFlushAndYield(['Count: 1']);
+      expect(Scheduler).toHaveYielded(['Count: 1']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 1')]);
       // Intentionally not updated because of [] deps:
       expect(counter.current.count).toBe(0);
@@ -1784,7 +1789,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       act(() => {
         counter.current.dispatch(INCREMENT);
       });
-      expect(Scheduler).toFlushAndYield(['Count: 1']);
+      expect(Scheduler).toHaveYielded(['Count: 1']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 1')]);
       expect(counter.current.count).toBe(1);
     });
@@ -1821,7 +1826,7 @@ describe('ReactHooksWithNoopRenderer', () => {
       act(() => {
         counter.current.dispatch(INCREMENT);
       });
-      expect(Scheduler).toFlushAndYield(['Count: 1']);
+      expect(Scheduler).toHaveYielded(['Count: 1']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 1')]);
       expect(counter.current.count).toBe(1);
       expect(totalRefUpdates).toBe(2);
@@ -1868,7 +1873,7 @@ describe('ReactHooksWithNoopRenderer', () => {
         updateB(3);
       });
 
-      expect(Scheduler).toFlushAndYield(['A: 2, B: 3, C: [not loaded]']);
+      expect(Scheduler).toHaveYielded(['A: 2, B: 3, C: [not loaded]']);
       expect(ReactNoop.getChildren()).toEqual([
         span('A: 2, B: 3, C: [not loaded]'),
       ]);
@@ -1929,7 +1934,7 @@ describe('ReactHooksWithNoopRenderer', () => {
         updateB(3);
         updateC(4);
       });
-      expect(Scheduler).toFlushAndYield(['A: 2, B: 3, C: 4']);
+      expect(Scheduler).toHaveYielded(['A: 2, B: 3, C: 4']);
       expect(ReactNoop.getChildren()).toEqual([span('A: 2, B: 3, C: 4')]);
       ReactNoop.render(<App loadC={false} />);
       expect(Scheduler).toFlushAndThrow(
@@ -2035,7 +2040,7 @@ describe('ReactHooksWithNoopRenderer', () => {
     act(() => {
       setCounter(2);
     });
-    expect(Scheduler).toFlushAndYield([
+    expect(Scheduler).toHaveYielded([
       'Render: 1',
       'Effect: 2',
       'Reducer: 2',
@@ -2074,7 +2079,7 @@ describe('ReactHooksWithNoopRenderer', () => {
     expect(ReactNoop).toMatchRenderedOutput('0');
 
     act(() => dispatch());
-    expect(Scheduler).toFlushAndYield(['Step: 5, Shadow: 5']);
+    expect(Scheduler).toHaveYielded(['Step: 5, Shadow: 5']);
     expect(ReactNoop).toMatchRenderedOutput('5');
   });
 
@@ -2113,7 +2118,8 @@ describe('ReactHooksWithNoopRenderer', () => {
 
       // Enqueuing this update forces the passive effect to be flushed --
       // updateCount(1) happens first, so 2 wins.
-      act(() => _updateCount(2));
+      // (use batchedUpdates to silence the act() warning)
+      ReactNoop.batchedUpdates(() => _updateCount(2));
       expect(Scheduler).toHaveYielded(['Will set count to 1']);
       expect(Scheduler).toFlushAndYield(['Count: 2']);
       expect(ReactNoop.getChildren()).toEqual([span('Count: 2')]);
diff --git a/packages/react-reconciler/src/__tests__/ReactIncrementalScheduling-test.internal.js b/packages/react-reconciler/src/__tests__/ReactIncrementalScheduling-test.internal.js
index 76158a89fe68b..ddc62c60aa6fa 100644
--- a/packages/react-reconciler/src/__tests__/ReactIncrementalScheduling-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactIncrementalScheduling-test.internal.js
@@ -104,14 +104,14 @@ describe('ReactIncrementalScheduling', () => {
       ReactNoop.renderToRootWithID(<Text text="b:1" />, 'b');
       ReactNoop.renderToRootWithID(<Text text="c:1" />, 'c');
     });
-    expect(Scheduler).toFlushAndYield(['a:1', 'b:1', 'c:1']);
+    expect(Scheduler).toHaveYielded(['a:1', 'b:1', 'c:1']);
 
     expect(ReactNoop.getChildrenAsJSX('a')).toEqual('a:1');
     expect(ReactNoop.getChildrenAsJSX('b')).toEqual('b:1');
     expect(ReactNoop.getChildrenAsJSX('c')).toEqual('c:1');
 
     // Schedule deferred work in the reverse order
-    ReactNoop.act(() => {
+    ReactNoop.batchedUpdates(() => {
       ReactNoop.renderToRootWithID(<Text text="c:2" />, 'c');
       ReactNoop.renderToRootWithID(<Text text="b:2" />, 'b');
     });
@@ -122,7 +122,7 @@ describe('ReactIncrementalScheduling', () => {
     expect(ReactNoop.getChildrenAsJSX('b')).toEqual('b:1');
     expect(ReactNoop.getChildrenAsJSX('c')).toEqual('c:2');
     // Schedule last bit of work, it will get processed the last
-    ReactNoop.act(() => {
+    ReactNoop.batchedUpdates(() => {
       ReactNoop.renderToRootWithID(<Text text="a:2" />, 'a');
     });
     // Keep performing work in the order it was scheduled
diff --git a/packages/react-reconciler/src/__tests__/ReactNoopRendererAct-test.js b/packages/react-reconciler/src/__tests__/ReactNoopRendererAct-test.js
index f3aba9b99881d..04f7febe843e7 100644
--- a/packages/react-reconciler/src/__tests__/ReactNoopRendererAct-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactNoopRendererAct-test.js
@@ -42,6 +42,7 @@ describe('ReactNoop.act()', () => {
         Scheduler.yieldValue('stage 1');
         await null;
         Scheduler.yieldValue('stage 2');
+        await null;
         setCtr(1);
       }
       React.useEffect(() => {
@@ -50,13 +51,9 @@ describe('ReactNoop.act()', () => {
       return ctr;
     }
     await ReactNoop.act(async () => {
-      ReactNoop.act(() => {
-        ReactNoop.render(<App />);
-      });
-      await null;
-      expect(Scheduler).toFlushAndYield(['stage 1']);
+      ReactNoop.render(<App />);
     });
-    expect(Scheduler).toHaveYielded(['stage 2']);
+    expect(Scheduler).toHaveYielded(['stage 1', 'stage 2']);
     expect(Scheduler).toFlushWithoutYielding();
     expect(ReactNoop.getChildren()).toEqual([{text: '1', hidden: false}]);
   });
diff --git a/packages/react-test-renderer/src/ReactTestRendererAct.js b/packages/react-test-renderer/src/ReactTestRendererAct.js
index 37ced3fb04c04..a3d1e4e9cc460 100644
--- a/packages/react-test-renderer/src/ReactTestRendererAct.js
+++ b/packages/react-test-renderer/src/ReactTestRendererAct.js
@@ -14,23 +14,42 @@ import {
 } from 'react-reconciler/inline.test';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import warningWithoutStack from 'shared/warningWithoutStack';
+import {warnAboutMissingMockScheduler} from 'shared/ReactFeatureFlags';
 import enqueueTask from 'shared/enqueueTask';
+import * as Scheduler from 'scheduler';
 
 const {ReactShouldWarnActingUpdates} = ReactSharedInternals;
 
 // this implementation should be exactly the same in
 // ReactTestUtilsAct.js, ReactTestRendererAct.js, createReactNoop.js
 
-// we track the 'depth' of the act() calls with this counter,
-// so we can tell if any async act() calls try to run in parallel.
-let actingUpdatesScopeDepth = 0;
+let hasWarnedAboutMissingMockScheduler = false;
+const flushWork =
+  Scheduler.unstable_flushWithoutYielding ||
+  function() {
+    if (warnAboutMissingMockScheduler === true) {
+      if (hasWarnedAboutMissingMockScheduler === false) {
+        warningWithoutStack(
+          null,
+          'Starting from React v17, the "scheduler" module will need to be mocked ' +
+            'to guarantee consistent behaviour across tests and browsers. To fix this, add the following ' +
+            "to the top of your tests, or in your framework's global config file -\n\n" +
+            'As an example, for jest - \n' +
+            "jest.mock('scheduler', () => require.requireActual('scheduler/unstable_mock'));\n\n" +
+            'For more info, visit https://fb.me/react-mock-scheduler',
+        );
+        hasWarnedAboutMissingMockScheduler = true;
+      }
+    }
+    while (flushPassiveEffects()) {}
+  };
 
-function flushEffectsAndMicroTasks(onDone: (err: ?Error) => void) {
+function flushWorkAndMicroTasks(onDone: (err: ?Error) => void) {
   try {
-    flushPassiveEffects();
+    flushWork();
     enqueueTask(() => {
-      if (flushPassiveEffects()) {
-        flushEffectsAndMicroTasks(onDone);
+      if (flushWork()) {
+        flushWorkAndMicroTasks(onDone);
       } else {
         onDone();
       }
@@ -40,6 +59,11 @@ function flushEffectsAndMicroTasks(onDone: (err: ?Error) => void) {
   }
 }
 
+// we track the 'depth' of the act() calls with this counter,
+// so we can tell if any async act() calls try to run in parallel.
+
+let actingUpdatesScopeDepth = 0;
+
 function act(callback: () => Thenable) {
   let previousActingUpdatesScopeDepth;
   if (__DEV__) {
@@ -100,7 +124,7 @@ function act(callback: () => Thenable) {
         called = true;
         result.then(
           () => {
-            flushEffectsAndMicroTasks((err: ?Error) => {
+            flushWorkAndMicroTasks((err: ?Error) => {
               onDone();
               if (err) {
                 reject(err);
@@ -128,7 +152,7 @@ function act(callback: () => Thenable) {
 
     // flush effects until none remain, and cleanup
     try {
-      while (flushPassiveEffects()) {}
+      flushWork();
       onDone();
     } catch (err) {
       onDone();
diff --git a/packages/scheduler/src/forks/SchedulerHostConfig.mock.js b/packages/scheduler/src/forks/SchedulerHostConfig.mock.js
index 0e2641c2c3be2..4ecd682ff8473 100644
--- a/packages/scheduler/src/forks/SchedulerHostConfig.mock.js
+++ b/packages/scheduler/src/forks/SchedulerHostConfig.mock.js
@@ -103,12 +103,15 @@ export function unstable_flushExpired() {
   }
 }
 
-export function unstable_flushWithoutYielding(): void {
+export function unstable_flushWithoutYielding(): boolean {
   if (isFlushing) {
     throw new Error('Already flushing work.');
   }
   isFlushing = true;
   try {
+    if (scheduledCallback === null) {
+      return false;
+    }
     while (scheduledCallback !== null) {
       const cb = scheduledCallback;
       scheduledCallback = null;
@@ -117,6 +120,7 @@ export function unstable_flushWithoutYielding(): void {
         scheduledCallbackExpiration <= currentTime;
       cb(didTimeout);
     }
+    return true;
   } finally {
     expectedNumberOfYields = -1;
     didStop = false;
diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index 1cabcfdd1b3b9..74b62b9821e9a 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -68,5 +68,8 @@ export const enableEventAPI = false;
 // New API for JSX transforms to target - https://github.com/reactjs/rfcs/pull/107
 export const enableJSXTransformAPI = false;
 
+// We will enforce mocking scheduler with scheduler/unstable_mock at some point. (v17?)
+// Till then, we warn about the missing mock, but still fallback to a sync mode compatible version
+export const warnAboutMissingMockScheduler = false;
 // Temporary flag to revert the fix in #15650
 export const revertPassiveEffectsChange = false;
diff --git a/packages/shared/forks/ReactFeatureFlags.native-fb.js b/packages/shared/forks/ReactFeatureFlags.native-fb.js
index 9aad62c48d515..8b899da6d62e5 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-fb.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-fb.js
@@ -32,6 +32,7 @@ export const warnAboutDeprecatedLifecycles = true;
 export const warnAboutDeprecatedSetNativeProps = true;
 export const enableEventAPI = false;
 export const enableJSXTransformAPI = false;
+export const warnAboutMissingMockScheduler = true;
 export const revertPassiveEffectsChange = false;
 
 // Only used in www builds.
diff --git a/packages/shared/forks/ReactFeatureFlags.native-oss.js b/packages/shared/forks/ReactFeatureFlags.native-oss.js
index 651a840b24981..c46a800a3a629 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-oss.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-oss.js
@@ -29,6 +29,7 @@ export const enableSchedulerDebugging = false;
 export const warnAboutDeprecatedSetNativeProps = false;
 export const enableEventAPI = false;
 export const enableJSXTransformAPI = false;
+export const warnAboutMissingMockScheduler = false;
 export const revertPassiveEffectsChange = false;
 
 // Only used in www builds.
diff --git a/packages/shared/forks/ReactFeatureFlags.persistent.js b/packages/shared/forks/ReactFeatureFlags.persistent.js
index 085a9bae85873..511a10fa10e14 100644
--- a/packages/shared/forks/ReactFeatureFlags.persistent.js
+++ b/packages/shared/forks/ReactFeatureFlags.persistent.js
@@ -29,6 +29,7 @@ export const enableSchedulerDebugging = false;
 export const warnAboutDeprecatedSetNativeProps = false;
 export const enableEventAPI = false;
 export const enableJSXTransformAPI = false;
+export const warnAboutMissingMockScheduler = true;
 export const revertPassiveEffectsChange = false;
 
 // Only used in www builds.
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
index f143adc2761e0..aa5f144f6cee5 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
@@ -29,6 +29,7 @@ export const enableSchedulerDebugging = false;
 export const warnAboutDeprecatedSetNativeProps = false;
 export const enableEventAPI = false;
 export const enableJSXTransformAPI = false;
+export const warnAboutMissingMockScheduler = false;
 export const revertPassiveEffectsChange = false;
 
 // Only used in www builds.
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
index 3f1f062aa683e..1e215516a263e 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
@@ -30,6 +30,7 @@ export const disableJavaScriptURLs = false;
 export const disableYielding = false;
 export const enableEventAPI = true;
 export const enableJSXTransformAPI = true;
+export const warnAboutMissingMockScheduler = true;
 
 // Only used in www builds.
 export function addUserTimingListener() {
diff --git a/packages/shared/forks/ReactFeatureFlags.www.js b/packages/shared/forks/ReactFeatureFlags.www.js
index 79d545431cbfa..40ca79e12865d 100644
--- a/packages/shared/forks/ReactFeatureFlags.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.www.js
@@ -72,6 +72,8 @@ export const enableEventAPI = true;
 
 export const enableJSXTransformAPI = true;
 
+export const warnAboutMissingMockScheduler = true;
+
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;
diff --git a/packages/shared/forks/Scheduler.umd.js b/packages/shared/forks/Scheduler.umd.js
index b1a77ce9232df..92738cbe9907b 100644
--- a/packages/shared/forks/Scheduler.umd.js
+++ b/packages/shared/forks/Scheduler.umd.js
@@ -28,6 +28,10 @@ const {
   unstable_LowPriority,
   unstable_IdlePriority,
   unstable_forceFrameRate,
+
+  // this doesn't actually exist on the scheduler, but it *does*
+  // on scheduler/unstable_mock, which we'll need inside act().
+  unstable_flushWithoutYielding,
 } = ReactInternals.Scheduler;
 
 export {
@@ -47,4 +51,5 @@ export {
   unstable_LowPriority,
   unstable_IdlePriority,
   unstable_forceFrameRate,
+  unstable_flushWithoutYielding,
 };
diff --git a/scripts/rollup/bundles.js b/scripts/rollup/bundles.js
index 5e2d7d9dfa0b1..446386ef7de5b 100644
--- a/scripts/rollup/bundles.js
+++ b/scripts/rollup/bundles.js
@@ -418,7 +418,14 @@ const bundles = [
 
   /******* React Scheduler Mock (experimental) *******/
   {
-    bundleTypes: [NODE_DEV, NODE_PROD, FB_WWW_DEV, FB_WWW_PROD],
+    bundleTypes: [
+      UMD_DEV,
+      UMD_PROD,
+      NODE_DEV,
+      NODE_PROD,
+      FB_WWW_DEV,
+      FB_WWW_PROD,
+    ],
     moduleType: ISOMORPHIC,
     entry: 'scheduler/unstable_mock',
     global: 'SchedulerMock',