diff --git a/types/react-dom/index.d.ts b/types/react-dom/index.d.ts
index 518c0c617c..a6ac5c9d11 100644
--- a/types/react-dom/index.d.ts
+++ b/types/react-dom/index.d.ts
@@ -1,10 +1,11 @@
-// Type definitions for React (react-dom) 16.0
+// Type definitions for React (react-dom) 16.8
 // Project: http://facebook.github.io/react/
 // Definitions by: Asana <https://asana.com>
 //                 AssureSign <http://www.assuresign.com>
 //                 Microsoft <https://microsoft.com>
 //                 MartynasZilinskas <https://github.com/MartynasZilinskas>
 //                 Josh Rutherford <https://github.com/theruther4d>
+//                 Jessica Franco <https://github.com/Jessidhia>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.8
 
diff --git a/types/react-dom/react-dom-tests.tsx b/types/react-dom/react-dom-tests.tsx
index c7ccb0052d..46274e3ffd 100644
--- a/types/react-dom/react-dom-tests.tsx
+++ b/types/react-dom/react-dom-tests.tsx
@@ -178,4 +178,23 @@ describe('React dom test utils', () => {
             shallowRenderer.getRenderOutput();
         });
     });
+
+    describe('act', () => {
+        it('accepts a sync callback that is void', () => {
+            ReactTestUtils.act(() => {});
+        });
+        it('rejects an async callback even if void', () => {
+            // $ExpectError
+            ReactTestUtils.act(async () => {});
+        });
+        it('rejects a callback that returns null', () => {
+            // $ExpectError
+            ReactTestUtils.act(() => null);
+        });
+        it('returns a Promise-like that errors out on use', () => {
+            const result = ReactTestUtils.act(() => {});
+            // $ExpectError
+            Promise.resolve(result);
+        });
+    });
 });
diff --git a/types/react-dom/test-utils/index.d.ts b/types/react-dom/test-utils/index.d.ts
index f60d088515..8730dc9447 100644
--- a/types/react-dom/test-utils/index.d.ts
+++ b/types/react-dom/test-utils/index.d.ts
@@ -278,3 +278,26 @@ export function findRenderedComponentWithType<T extends Component<any>, C extend
  * Call this in your tests to create a shallow renderer.
  */
 export function createRenderer(): ShallowRenderer;
+
+/**
+ * Wrap any code rendering and triggering updates to your components into `act()` calls.
+ *
+ * Ensures that the behavior in your tests matches what happens in the browser
+ * more closely by executing pending `useEffect`s before returning. This also
+ * reduces the amount of re-renders done.
+ *
+ * @param callback A synchronous, void callback that will execute as a single, complete React commit.
+ *
+ * @see https://reactjs.org/blog/2019/02/06/react-v16.8.0.html#testing-hooks
+ */
+// the "void | undefined" is here to forbid any sneaky "Promise" returns.
+// the actual return value is always a "DebugPromiseLike",
+// but having an "| {}" makes it harder to accidentally use.
+export function act(callback: () => void | undefined): DebugPromiseLike | {};
+
+// Intentionally doesn't extend PromiseLike<never>.
+// Ideally this should be as hard to accidentally use as possible.
+export interface DebugPromiseLike {
+    // the actual then() in here is 0-ary, but that doesn't count as a PromiseLike.
+    then(onfulfilled: (value: never) => never, onrejected: (reason: never) => never): never;
+}
diff --git a/types/react-dom/tsconfig.json b/types/react-dom/tsconfig.json
index f63325026f..6d7d943c8b 100644
--- a/types/react-dom/tsconfig.json
+++ b/types/react-dom/tsconfig.json
@@ -14,7 +14,7 @@
         ],
         "noImplicitAny": true,
         "noImplicitThis": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "strictFunctionTypes": true,
         "baseUrl": "../",
         "typeRoots": [
diff --git a/types/react-test-renderer/index.d.ts b/types/react-test-renderer/index.d.ts
index c147d5c190..abdae1c908 100644
--- a/types/react-test-renderer/index.d.ts
+++ b/types/react-test-renderer/index.d.ts
@@ -1,9 +1,10 @@
-// Type definitions for react-test-renderer 16.0
+// Type definitions for react-test-renderer 16.8
 // Project: https://facebook.github.io/react/
 // Definitions by: Arvitaly <https://github.com/arvitaly>
 //                 Lochbrunner <https://github.com/lochbrunner>
 //                 John Reilly <https://github.com/johnnyreilly>
 //                 John Gozde <https://github.com/jgoz>
+//                 Jessica Franco <https://github.com/Jessidhia>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.8
 
@@ -50,3 +51,26 @@ export interface TestRendererOptions {
     createNodeMock(element: ReactElement<any>): any;
 }
 export function create(nextElement: ReactElement<any>, options?: TestRendererOptions): ReactTestRenderer;
+
+/**
+ * Wrap any code rendering and triggering updates to your components into `act()` calls.
+ *
+ * Ensures that the behavior in your tests matches what happens in the browser
+ * more closely by executing pending `useEffect`s before returning. This also
+ * reduces the amount of re-renders done.
+ *
+ * @param callback A synchronous, void callback that will execute as a single, complete React commit.
+ *
+ * @see https://reactjs.org/blog/2019/02/06/react-v16.8.0.html#testing-hooks
+ */
+// the "void | undefined" is here to forbid any sneaky "Promise" returns.
+// the actual return value is always a "DebugPromiseLike",
+// but having an "| {}" makes it harder to accidentally use.
+export function act(callback: () => void | undefined): DebugPromiseLike | {};
+
+// Intentionally doesn't extend PromiseLike<never>.
+// Ideally this should be as hard to accidentally use as possible.
+export interface DebugPromiseLike {
+    // the actual then() in here is 0-ary, but that doesn't count as a PromiseLike.
+    then(onfulfilled: (value: never) => never, onrejected: (reason: never) => never): never;
+}
diff --git a/types/react-test-renderer/react-test-renderer-tests.ts b/types/react-test-renderer/react-test-renderer-tests.ts
index 83dc510e2b..48d861cf62 100644
--- a/types/react-test-renderer/react-test-renderer-tests.ts
+++ b/types/react-test-renderer/react-test-renderer-tests.ts
@@ -1,5 +1,5 @@
-import * as React from "react";
-import { create, ReactTestInstance } from "react-test-renderer";
+import React = require("react");
+import { act, create, ReactTestInstance } from "react-test-renderer";
 import { createRenderer } from 'react-test-renderer/shallow';
 
 class TestComponent extends React.Component { }
@@ -66,3 +66,12 @@ const shallowRenderer = createRenderer();
 shallowRenderer.render(component);
 shallowRenderer.getRenderOutput();
 shallowRenderer.getMountedInstance();
+
+// Only synchronous, void callbacks are acceptable for act()
+act(() => {});
+// $ExpectError
+act(async () => {});
+// $ExpectError
+act(() => null);
+// $ExpectError
+Promise.resolve(act(() => {}));
diff --git a/types/react/index.d.ts b/types/react/index.d.ts
index 929a698775..9e46bc5ca9 100644
--- a/types/react/index.d.ts
+++ b/types/react/index.d.ts
@@ -783,11 +783,9 @@ declare namespace React {
     // TODO (TypeScript 3.0): ReadonlyArray<unknown>
     type DependencyList = ReadonlyArray<any>;
 
-    // NOTE: Currently, in alpha.0, the effect callbacks are actually allowed to return anything,
-    // but functions are treated specially. The next version published with hooks will warn if you actually
-    // return anything besides `void` or a callback. Async effects need to call an async function inside
-    // them.
-    type EffectCallback = () => (void | (() => void));
+    // NOTE: callbacks are _only_ allowed to return either void, or a destructor.
+    // The destructor is itself only allowed to return void.
+    type EffectCallback = () => (void | (() => void | undefined));
 
     interface MutableRefObject<T> {
         current: T;
@@ -853,12 +851,16 @@ declare namespace React {
      * @version 16.8.0
      * @see https://reactjs.org/docs/hooks-reference.html#usereducer
      */
+
     // I'm not sure if I keep this 2-ary or if I make it (2,3)-ary; it's currently (2,3)-ary.
     // The Flow types do have an overload for 3-ary invocation with undefined initializer.
-    // NOTE: the documentation or any alphas aren't updated, this is current for master.
-    // NOTE 2: without the ReducerState indirection, TypeScript would reduce S to be the most common
+
+    // NOTE: without the ReducerState indirection, TypeScript would reduce S to be the most common
     // supertype between the reducer's return type and the initialState (or the initializer's return type),
     // which would prevent autocompletion from ever working.
+
+    // TODO: double-check if this weird overload logic is necessary. It is possible it's either a bug
+    // in older versions, or a regression in newer versions of the typescript completion service.
     function useReducer<R extends Reducer<any, any>>(
         reducer: R,
         initialState: ReducerState<R>,
diff --git a/types/react/test/hooks.tsx b/types/react/test/hooks.tsx
index 12fcc28222..26ff634e89 100644
--- a/types/react/test/hooks.tsx
+++ b/types/react/test/hooks.tsx
@@ -120,6 +120,23 @@ function useEveryHook(ref: React.Ref<{ id: number }>|undefined): () => boolean {
         setState(reducerState.age);
     }, []);
 
+    // effects are only allowed to either be actually void or return actually void functions
+    React.useEffect(() => () => {});
+    // indistinguishable
+    React.useEffect(() => () => undefined);
+    // $ExpectError
+    React.useEffect(() => null);
+    // $ExpectError
+    React.useEffect(() => Math.random() ? null : undefined);
+    // $ExpectError
+    React.useEffect(() => () => null);
+    // $ExpectError
+    React.useEffect(() => () => Math.random() ? null : undefined);
+    // $ExpectError
+    React.useEffect(() => async () => {});
+    // $ExpectError
+    React.useEffect(async () => () => {});
+
     React.useDebugValue(id, value => value.toFixed());
     React.useDebugValue(id);