diff --git a/.eslintrc.js b/.eslintrc.js
index 51c9395f73c..cd308d281a7 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -19,6 +19,7 @@ module.exports = {
     'no-for-of-loops',
     'react',
     'react-internal',
+    'flowtype',
   ],
 
   parser: 'espree',
@@ -89,6 +90,7 @@ module.exports = {
     'react-internal/no-primitive-constructors': ERROR,
     'react-internal/no-to-warn-dev-within-to-throw': ERROR,
     'react-internal/warning-and-invariant-args': ERROR,
+    'flowtype/no-types-missing-file-annotation': ERROR,
   },
 
   overrides: [
diff --git a/packages/events/EventPropagators.js b/packages/events/EventPropagators.js
index 5163c2b7a3f..a53d33c7005 100644
--- a/packages/events/EventPropagators.js
+++ b/packages/events/EventPropagators.js
@@ -3,8 +3,15 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
+import type {Fiber} from 'react-reconciler/src/ReactFiber';
+import type {
+  ReactSyntheticEvent,
+  DispatchConfig,
+} from './ReactSyntheticEventType';
 import {
   getParentInstance,
   traverseTwoPhase,
@@ -106,24 +113,33 @@ function accumulateDispatches(inst, ignoredDirection, event) {
  * `dispatchMarker`.
  * @param {SyntheticEvent} event
  */
-function accumulateDirectDispatchesSingle(event) {
+function accumulateDirectDispatchesSingle(event: ReactSyntheticEvent) {
   if (event && event.dispatchConfig.registrationName) {
     accumulateDispatches(event._targetInst, null, event);
   }
 }
 
-export function accumulateTwoPhaseDispatches(events) {
+export function accumulateTwoPhaseDispatches(
+  events: Array<ReactSyntheticEvent>,
+) {
   forEachAccumulated(events, accumulateTwoPhaseDispatchesSingle);
 }
 
-export function accumulateTwoPhaseDispatchesSkipTarget(events) {
+export function accumulateTwoPhaseDispatchesSkipTarget(
+  events: Array<ReactSyntheticEvent>,
+) {
   forEachAccumulated(events, accumulateTwoPhaseDispatchesSingleSkipTarget);
 }
 
-export function accumulateEnterLeaveDispatches(leave, enter, from, to) {
+export function accumulateEnterLeaveDispatches(
+  leave: DispatchConfig,
+  enter: DispatchConfig,
+  from: Fiber,
+  to: Fiber,
+) {
   traverseEnterLeave(from, to, accumulateDispatches, leave, enter);
 }
 
-export function accumulateDirectDispatches(events) {
+export function accumulateDirectDispatches(events: Array<ReactSyntheticEvent>) {
   forEachAccumulated(events, accumulateDirectDispatchesSingle);
 }
diff --git a/packages/events/ReactSyntheticEventType.js b/packages/events/ReactSyntheticEventType.js
index 07ddf270f2e..849dcb9b3e5 100644
--- a/packages/events/ReactSyntheticEventType.js
+++ b/packages/events/ReactSyntheticEventType.js
@@ -30,4 +30,7 @@ export type ReactSyntheticEvent = {
     nativeEventTarget: EventTarget,
   ) => ReactSyntheticEvent,
   isPersistent: () => boolean,
+  _targetInst: Fiber,
+  _dispatchListeners: ReactSyntheticEvent | Array<ReactSyntheticEvent>,
+  _dispatchInstances: Fiber | Array<Fiber>,
 } & SyntheticEvent<>;
diff --git a/packages/react-art/src/ReactARTHostConfig.js b/packages/react-art/src/ReactARTHostConfig.js
index 034b2d7290a..5bc78277f8b 100644
--- a/packages/react-art/src/ReactARTHostConfig.js
+++ b/packages/react-art/src/ReactARTHostConfig.js
@@ -421,6 +421,6 @@ export function unhideInstance(instance, props) {
   }
 }
 
-export function unhideTextInstance(textInstance, text): void {
+export function unhideTextInstance(textInstance, text) {
   // Noop
 }
diff --git a/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js b/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
index 8879bca9987..f876196d049 100644
--- a/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
+++ b/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @emails react-core
+ * @flow
  */
 
 'use strict';
diff --git a/packages/react-dom/src/__tests__/ReactCompositeComponent-test.js b/packages/react-dom/src/__tests__/ReactCompositeComponent-test.js
index 43ac793cf59..69451536629 100644
--- a/packages/react-dom/src/__tests__/ReactCompositeComponent-test.js
+++ b/packages/react-dom/src/__tests__/ReactCompositeComponent-test.js
@@ -9,6 +9,8 @@
 
 'use strict';
 
+import shallowEqual from 'shared/shallowEqual';
+
 let ChildUpdates;
 let MorphingComponent;
 let React;
@@ -19,41 +21,6 @@ let ReactTestUtils;
 let PropTypes;
 
 describe('ReactCompositeComponent', () => {
-  const hasOwnProperty = Object.prototype.hasOwnProperty;
-
-  /**
-   * Performs equality by iterating through keys on an object and returning false
-   * when any key has values which are not strictly equal between the arguments.
-   * Returns true when the values of all keys are strictly equal.
-   */
-  function shallowEqual(objA: mixed, objB: mixed): boolean {
-    if (Object.is(objA, objB)) {
-      return true;
-    }
-    if (
-      typeof objA !== 'object' ||
-      objA === null ||
-      typeof objB !== 'object' ||
-      objB === null
-    ) {
-      return false;
-    }
-    const keysA = Object.keys(objA);
-    const keysB = Object.keys(objB);
-    if (keysA.length !== keysB.length) {
-      return false;
-    }
-    for (let i = 0; i < keysA.length; i++) {
-      if (
-        !hasOwnProperty.call(objB, keysA[i]) ||
-        !Object.is(objA[keysA[i]], objB[keysA[i]])
-      ) {
-        return false;
-      }
-    }
-    return true;
-  }
-
   function shallowCompare(instance, nextProps, nextState) {
     return (
       !shallowEqual(instance.props, nextProps) ||
diff --git a/packages/react-dom/src/events/BeforeInputEventPlugin.js b/packages/react-dom/src/events/BeforeInputEventPlugin.js
index 6895e0dc625..d7105bc47a5 100644
--- a/packages/react-dom/src/events/BeforeInputEventPlugin.js
+++ b/packages/react-dom/src/events/BeforeInputEventPlugin.js
@@ -3,6 +3,8 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 import type {TopLevelType} from 'events/TopLevelEventTypes';
diff --git a/packages/react-dom/src/shared/DOMNamespaces.js b/packages/react-dom/src/shared/DOMNamespaces.js
index ca0e8fb3770..f19aa6e1ff8 100644
--- a/packages/react-dom/src/shared/DOMNamespaces.js
+++ b/packages/react-dom/src/shared/DOMNamespaces.js
@@ -3,6 +3,8 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 const HTML_NAMESPACE = 'http://www.w3.org/1999/xhtml';
diff --git a/packages/react-dom/src/shared/assertValidProps.js b/packages/react-dom/src/shared/assertValidProps.js
index 9fe255b58da..59045e80e36 100644
--- a/packages/react-dom/src/shared/assertValidProps.js
+++ b/packages/react-dom/src/shared/assertValidProps.js
@@ -3,6 +3,8 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 import invariant from 'shared/invariant';
@@ -31,7 +33,7 @@ function assertValidProps(tag: string, props: ?Object) {
       '%s is a void element tag and must neither have `children` nor ' +
         'use `dangerouslySetInnerHTML`.%s',
       tag,
-      __DEV__ ? ReactDebugCurrentFrame.getStackAddendum() : '',
+      __DEV__ && ReactDebugCurrentFrame ? ReactDebugCurrentFrame.getStackAddendum() : '',
     );
   }
   if (props.dangerouslySetInnerHTML != null) {
@@ -63,7 +65,7 @@ function assertValidProps(tag: string, props: ?Object) {
     'The `style` prop expects a mapping from style properties to values, ' +
       "not a string. For example, style={{marginRight: spacing + 'em'}} when " +
       'using JSX.%s',
-    __DEV__ ? ReactDebugCurrentFrame.getStackAddendum() : '',
+    __DEV__ && ReactDebugCurrentFrame ? ReactDebugCurrentFrame.getStackAddendum() : '',
   );
 }
 
diff --git a/packages/react-native-renderer/src/__mocks__/deepDiffer.js b/packages/react-native-renderer/src/__mocks__/deepDiffer.js
index a4ede524658..88111330ed4 100644
--- a/packages/react-native-renderer/src/__mocks__/deepDiffer.js
+++ b/packages/react-native-renderer/src/__mocks__/deepDiffer.js
@@ -3,6 +3,8 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 'use strict';
diff --git a/packages/react/src/ReactLazy.js b/packages/react/src/ReactLazy.js
index 28415fcf265..4b8eb08df29 100644
--- a/packages/react/src/ReactLazy.js
+++ b/packages/react/src/ReactLazy.js
@@ -3,13 +3,17 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 import type {LazyComponent, Thenable} from 'shared/ReactLazyComponent';
 
 import {REACT_LAZY_TYPE} from 'shared/ReactSymbols';
 
-export function lazy<T, R>(ctor: () => Thenable<T, R>): LazyComponent<T> {
+export function lazy<T>(
+  ctor: () => Thenable<{default: T}, mixed>,
+): LazyComponent<T> {
   return {
     $$typeof: REACT_LAZY_TYPE,
     _ctor: ctor,
diff --git a/packages/react/src/forwardRef.js b/packages/react/src/forwardRef.js
index c2617e4eef9..63fd7ae495c 100644
--- a/packages/react/src/forwardRef.js
+++ b/packages/react/src/forwardRef.js
@@ -3,6 +3,8 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 import {REACT_FORWARD_REF_TYPE, REACT_MEMO_TYPE} from 'shared/ReactSymbols';
diff --git a/packages/react/src/memo.js b/packages/react/src/memo.js
index 0fda3a8f67e..83f02e77462 100644
--- a/packages/react/src/memo.js
+++ b/packages/react/src/memo.js
@@ -3,6 +3,8 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 import {REACT_MEMO_TYPE} from 'shared/ReactSymbols';
diff --git a/packages/scheduler/src/Scheduler.js b/packages/scheduler/src/Scheduler.js
index c815d0071bd..f080a503839 100644
--- a/packages/scheduler/src/Scheduler.js
+++ b/packages/scheduler/src/Scheduler.js
@@ -98,7 +98,7 @@ function flushFirstCallback() {
   // A callback may return a continuation. The continuation should be scheduled
   // with the same priority and expiration as the just-finished callback.
   if (typeof continuationCallback === 'function') {
-    var continuationNode: CallbackNode = {
+    var continuationNode = {
       callback: continuationCallback,
       priorityLevel,
       expirationTime,
diff --git a/packages/scheduler/src/__tests__/SchedulerDOM-test.js b/packages/scheduler/src/__tests__/SchedulerDOM-test.js
index 562113f65bb..9c7b1b37547 100644
--- a/packages/scheduler/src/__tests__/SchedulerDOM-test.js
+++ b/packages/scheduler/src/__tests__/SchedulerDOM-test.js
@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @emails react-core
+ * @flow
  */
 
 'use strict';
diff --git a/packages/shared/ReactLazyComponent.js b/packages/shared/ReactLazyComponent.js
index f3042ab11ae..2033a7fad9e 100644
--- a/packages/shared/ReactLazyComponent.js
+++ b/packages/shared/ReactLazyComponent.js
@@ -14,7 +14,7 @@ export type Thenable<T, R> = {
 export type LazyComponent<T> = {
   $$typeof: Symbol | number,
   _ctor: () => Thenable<{default: T}, mixed>,
-  _status: 0 | 1 | 2,
+  _status: -1 | 0 | 1 | 2, // TODO: what status is -1?
   _result: any,
 };
 
diff --git a/packages/shared/forks/invokeGuardedCallbackImpl.www.js b/packages/shared/forks/invokeGuardedCallbackImpl.www.js
index c624679a8a7..2a0452e6065 100644
--- a/packages/shared/forks/invokeGuardedCallbackImpl.www.js
+++ b/packages/shared/forks/invokeGuardedCallbackImpl.www.js
@@ -3,11 +3,14 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 import invariant from 'shared/invariant';
 
 // Provided by www
+// $FlowIssue
 const ReactFbErrorUtils = require('ReactFbErrorUtils');
 invariant(
   typeof ReactFbErrorUtils.invokeGuardedCallback === 'function',