diff --git a/Libraries/Utilities/Dimensions.js b/Libraries/Utilities/Dimensions.js
index 3bfaf9c52..80d564a2b 100644
--- a/Libraries/Utilities/Dimensions.js
+++ b/Libraries/Utilities/Dimensions.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule Dimensions
+ * @flow
  */
 'use strict';
 
@@ -22,7 +23,7 @@ class Dimensions {
    *
    * @param {object} dims Simple string-keyed object of dimensions to set
    */
-  static set(dims) {
+  static set(dims: {[key:string]: any}): bool {
     Object.assign(dimensions, dims);
     return true;
   }
@@ -40,7 +41,7 @@ class Dimensions {
    * @param {string} dim Name of dimension as defined when calling `set`.
    * @returns {Object?} Value for the dimension.
    */
-  static get(dim) {
+  static get(dim: string): Object {
     invariant(dimensions[dim], 'No dimension set for key ' + dim);
     return dimensions[dim];
   }
diff --git a/Libraries/Utilities/MessageQueue.js b/Libraries/Utilities/MessageQueue.js
index 81091a54f..9d6eb867e 100644
--- a/Libraries/Utilities/MessageQueue.js
+++ b/Libraries/Utilities/MessageQueue.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule MessageQueue
+ * @flow
  */
 'use strict';
 var ErrorUtils = require('ErrorUtils');
@@ -18,6 +19,18 @@ var JSTimersExecution = require('JSTimersExecution');
 
 var INTERNAL_ERROR = 'Error in MessageQueue implementation';
 
+type ModulesConfig = {
+  [key:string]: {
+    moduleID: number;
+    methods: {[key:string]: {
+      methodID: number;
+    }};
+  }
+}
+
+type NameToID = {[key:string]: number}
+type IDToName = {[key:number]: string}
+
 /**
  * So as not to confuse static build system.
  */
@@ -45,7 +58,11 @@ var jsCall = function(module, methodName, params) {
  * efficient numeric IDs.
  * @class MessageQueue
  */
-var MessageQueue = function(remoteModulesConfig, localModulesConfig, customRequire) {
+var MessageQueue = function(
+  remoteModulesConfig: ModulesConfig,
+  localModulesConfig: ModulesConfig,
+  customRequire: (id: string) => any
+) {
   this._requireFunc = customRequire || requireFunc;
   this._initBookeeping();
   this._initNamingMap(remoteModulesConfig, localModulesConfig);
@@ -128,7 +145,10 @@ var MessageQueueMixin = {
    * @param {object} remoteModulesConfig Configuration of modules and their
    * methods.
    */
-  _initNamingMap: function(remoteModulesConfig, localModulesConfig) {
+  _initNamingMap: function(
+    remoteModulesConfig: ModulesConfig,
+    localModulesConfig: ModulesConfig
+  ) {
     this._remoteModuleNameToModuleID = {};
     this._remoteModuleIDToModuleName = {};         // Reverse
 
@@ -142,11 +162,11 @@ var MessageQueueMixin = {
     this._localModuleNameToMethodIDToName = {};   // Reverse
 
     function fillMappings(
-      modulesConfig,
-      moduleNameToModuleID,
-      moduleIDToModuleName,
-      moduleNameToMethodNameToID,
-      moduleNameToMethodIDToName
+      modulesConfig: ModulesConfig,
+      moduleNameToModuleID: NameToID,
+      moduleIDToModuleName: IDToName,
+      moduleNameToMethodNameToID: {[key:string]: NameToID},
+      moduleNameToMethodIDToName: {[key:string]: IDToName}
     ) {
       for (var moduleName in modulesConfig) {
         var moduleConfig = modulesConfig[moduleName];
diff --git a/Libraries/Utilities/PixelRatio.js b/Libraries/Utilities/PixelRatio.js
index 4770937b1..f8e23398a 100644
--- a/Libraries/Utilities/PixelRatio.js
+++ b/Libraries/Utilities/PixelRatio.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule PixelRatio
+ * @flow
  */
 'use strict';
 
@@ -52,7 +53,7 @@ class PixelRatio {
    *   - PixelRatio.get() === 3
    *     - iPhone 6 plus
    */
-  static get() {
+  static get(): number {
     return Dimensions.get('window').scale;
   }
 }
diff --git a/Libraries/Utilities/Platform.ios.js b/Libraries/Utilities/Platform.ios.js
index 515e7d288..d02e6fde2 100644
--- a/Libraries/Utilities/Platform.ios.js
+++ b/Libraries/Utilities/Platform.ios.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule Platform
+ * @flow
  */
 
 'use strict';
diff --git a/Libraries/Utilities/PushNotificationIOS.js b/Libraries/Utilities/PushNotificationIOS.js
index fe77c69b3..ed581ba20 100644
--- a/Libraries/Utilities/PushNotificationIOS.js
+++ b/Libraries/Utilities/PushNotificationIOS.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule PushNotificationIOS
+ * @flow
  */
 'use strict';
 
@@ -23,6 +24,10 @@ var _notifHandlers = {};
 var DEVICE_NOTIF_EVENT = 'remoteNotificationReceived';
 
 class PushNotificationIOS {
+  _data: Object;
+  _alert: string | Object;
+  _sound: string;
+  _badgeCount: number;
 
   static addEventListener(type, handler) {
     _notifHandlers[handler] = RCTDeviceEventEmitter.addListener(
@@ -68,24 +73,24 @@ class PushNotificationIOS {
     });
   }
 
-  getMessage() {
+  getMessage(): ?string | ?Object {
     // alias because "alert" is an ambiguous name
     return this._alert;
   }
 
-  getSound() {
+  getSound(): ?string {
     return this._sound;
   }
 
-  getAlert() {
+  getAlert(): ?string | ?Object {
     return this._alert;
   }
 
-  getBadgeCount() {
+  getBadgeCount(): ?number {
     return this._badgeCount;
   }
 
-  getData() {
+  getData(): ?Object {
     return this._data;
   }
 }
diff --git a/Libraries/Utilities/RCTLog.js b/Libraries/Utilities/RCTLog.js
index 8955382e0..e5a8db49c 100644
--- a/Libraries/Utilities/RCTLog.js
+++ b/Libraries/Utilities/RCTLog.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule RCTLog
+ * @flow
  */
  /* globals nativeLoggingHook */
 'use strict';
@@ -31,7 +32,7 @@ class RCTLog {
       logFn,
       'Level "' + level + '" not one of ' + Object.keys(levelsMap)
     );
-    if (typeof nativeLoggingHook === 'undefined') {
+    if (typeof global.nativeLoggingHook === 'undefined') {
       // We already printed in xcode, so only log here if using a js debugger
       console[logFn].apply(console, args);
     }
diff --git a/Libraries/Utilities/RCTRenderingPerf.js b/Libraries/Utilities/RCTRenderingPerf.js
index 5662902f0..c6466aa64 100644
--- a/Libraries/Utilities/RCTRenderingPerf.js
+++ b/Libraries/Utilities/RCTRenderingPerf.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule RCTRenderingPerf
+ * @flow
  */
 'use strict';
 
@@ -15,6 +16,11 @@ var ReactPerf = require('ReactPerf');
 
 var invariant = require('invariant');
 
+type perfModule = {
+  start: () => void;
+  stop: () => void;
+}
+
 var perfModules = [];
 var enabled = false;
 
@@ -58,7 +64,7 @@ var RCTRenderingPerf = {
     perfModules.forEach((module) => module.stop());
   },
 
-  register: function(module) {
+  register: function(module: perfModule) {
     invariant(
       typeof module.start === 'function',
       'Perf module should have start() function'
diff --git a/Libraries/Utilities/TimerMixin.js b/Libraries/Utilities/TimerMixin.js
index 2c570660a..0fc68457d 100644
--- a/Libraries/Utilities/TimerMixin.js
+++ b/Libraries/Utilities/TimerMixin.js
@@ -7,9 +7,13 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule TimerMixin
+ * @flow
  */
 'use strict';
 
+var setImmediate = require('setImmediate');
+var clearImmediate = require('clearImmediate');
+
 /**
  * Using bare setTimeout, setInterval, setImmediate and
  * requestAnimationFrame calls is very dangerous because if you forget to cancel
@@ -35,7 +39,10 @@
  */
 
  var setter = function(setter, clearer, array) {
-   return function(callback, delta) {
+   return function(
+     callback: () => void,
+     delta: number
+   ): number {
      var id = setter(() => {
        clearer.call(this, id);
        callback.apply(this, arguments);
@@ -51,7 +58,7 @@
  };
 
  var clearer = function(clearer, array) {
-   return function(id) {
+   return function(id: number) {
      if (this[array]) {
        var index = this[array].indexOf(id);
        if (index !== -1) {
@@ -75,8 +82,8 @@
  var _setImmediate = setter(setImmediate, _clearImmediate, _immediates);
 
  var _rafs = 'TimerMixin_rafs';
- var _cancelAnimationFrame = clearer(cancelAnimationFrame, _rafs);
- var _requestAnimationFrame = setter(requestAnimationFrame, _cancelAnimationFrame, _rafs);
+ var _cancelAnimationFrame = clearer(window.cancelAnimationFrame, _rafs);
+ var _requestAnimationFrame = setter(window.requestAnimationFrame, _cancelAnimationFrame, _rafs);
 
 var TimerMixin = {
   componentWillUnmount: function() {
diff --git a/Libraries/Utilities/createStrictShapeTypeChecker.js b/Libraries/Utilities/createStrictShapeTypeChecker.js
index 0812aa6f4..cd0d5757a 100644
--- a/Libraries/Utilities/createStrictShapeTypeChecker.js
+++ b/Libraries/Utilities/createStrictShapeTypeChecker.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule createStrictShapeTypeChecker
+ * @flow
  */
 'use strict';
 
@@ -15,8 +16,10 @@ var ReactPropTypeLocationNames = require('ReactPropTypeLocationNames');
 var invariant = require('invariant');
 var merge = require('merge');
 
-function createStrictShapeTypeChecker(shapeTypes) {
-  function checkType(isRequired, props, propName, componentName, location) {
+function createStrictShapeTypeChecker(
+  shapeTypes: {[key: string]: ReactPropsCheckType}
+): ReactPropsChainableTypeChecker {
+  function checkType(isRequired, props, propName, componentName, location?) {
     if (!props[propName]) {
       if (isRequired) {
         invariant(
@@ -29,7 +32,8 @@ function createStrictShapeTypeChecker(shapeTypes) {
     }
     var propValue = props[propName];
     var propType = typeof propValue;
-    var locationName = ReactPropTypeLocationNames[location];
+    var locationName =
+      location && ReactPropTypeLocationNames[location] || '(unknown)';
     if (propType !== 'object') {
       invariant(
         false,
@@ -57,11 +61,17 @@ function createStrictShapeTypeChecker(shapeTypes) {
           error.message +
             `\nBad object: ` + JSON.stringify(props[propName], null, '  ')
         );
-        return error;
       }
     }
   }
-  var chainedCheckType = checkType.bind(null, false);
+  function chainedCheckType(
+    props: {[key: string]: any},
+    propName: string,
+    componentName: string,
+    location?: string
+  ): ?Error {
+    return checkType(false, props, propName, componentName, location);
+  }
   chainedCheckType.isRequired = checkType.bind(null, true);
   return chainedCheckType;
 }
diff --git a/Libraries/Utilities/deepFreezeAndThrowOnMutationInDev.js b/Libraries/Utilities/deepFreezeAndThrowOnMutationInDev.js
index e5530204e..9263116ed 100644
--- a/Libraries/Utilities/deepFreezeAndThrowOnMutationInDev.js
+++ b/Libraries/Utilities/deepFreezeAndThrowOnMutationInDev.js
@@ -7,8 +7,11 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule deepFreezeAndThrowOnMutationInDev
+ * @flow
  */
 
+'use strict';
+
 /**
  * If your application is accepting different values for the same field over
  * time and is doing a diff on them, you can either (1) create a copy or
@@ -26,7 +29,7 @@
  * Freezing the object and adding the throw mechanism is expensive and will
  * only be used in DEV.
  */
-function deepFreezeAndThrowOnMutationInDev(object) {
+function deepFreezeAndThrowOnMutationInDev(object: Object) {
   if (__DEV__) {
     if (typeof object !== 'object' ||
         object === null ||
diff --git a/Libraries/Utilities/differ/insetsDiffer.js b/Libraries/Utilities/differ/insetsDiffer.js
index 887a482a1..b0a409470 100644
--- a/Libraries/Utilities/differ/insetsDiffer.js
+++ b/Libraries/Utilities/differ/insetsDiffer.js
@@ -7,9 +7,17 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule insetsDiffer
+ * @flow
  */
 'use strict';
 
+type Inset = {
+  top: ?number;
+  left: ?number;
+  right: ?number;
+  bottom: ?number;
+}
+
 var dummyInsets = {
 	top: undefined,
 	left: undefined,
@@ -17,7 +25,10 @@ var dummyInsets = {
 	bottom: undefined,
 };
 
-var insetsDiffer = function(one, two) {
+var insetsDiffer = function(
+  one: ?Inset,
+  two: ?Inset
+): bool {
   one = one || dummyInsets;
   two = two || dummyInsets;
   return one !== two && (
diff --git a/Libraries/Utilities/differ/pointsDiffer.js b/Libraries/Utilities/differ/pointsDiffer.js
index cd2c558d2..c4939199f 100644
--- a/Libraries/Utilities/differ/pointsDiffer.js
+++ b/Libraries/Utilities/differ/pointsDiffer.js
@@ -7,12 +7,18 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule pointsDiffer
+ * @flow
  */
 'use strict';
 
+type Point = {
+  x: ?number;
+  y: ?number;
+}
+
 var dummyPoint = {x: undefined, y: undefined};
 
-var pointsDiffer = function(one, two) {
+var pointsDiffer = function(one: ?Point, two: ?Point): bool {
   one = one || dummyPoint;
   two = two || dummyPoint;
   return one !== two && (
diff --git a/Libraries/Utilities/groupByEveryN.js b/Libraries/Utilities/groupByEveryN.js
index d1127c1d1..a8b353ad6 100644
--- a/Libraries/Utilities/groupByEveryN.js
+++ b/Libraries/Utilities/groupByEveryN.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule groupByEveryN
+ * @flow
  */
 
 /**
@@ -26,7 +27,7 @@
  */
 'use strict';
 
-function groupByEveryN(array, n) {
+function groupByEveryN<T>(array: Array<T>, n: number): Array<Array<?T>> {
   var result = [];
   var temp = [];
 
diff --git a/Libraries/Utilities/logError.js b/Libraries/Utilities/logError.js
index 1a0be1a13..aac41be0b 100644
--- a/Libraries/Utilities/logError.js
+++ b/Libraries/Utilities/logError.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule logError
+ * @flow
  */
 'use strict';
 
diff --git a/Libraries/Utilities/mergeFast.js b/Libraries/Utilities/mergeFast.js
index 8923167e7..e55ac76ed 100644
--- a/Libraries/Utilities/mergeFast.js
+++ b/Libraries/Utilities/mergeFast.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule mergeFast
+ * @flow
  */
 'use strict';
 
@@ -19,7 +20,7 @@
  * @return {object} Merging of two objects, including prototype
  * inherited properties.
  */
-var mergeFast = function(one, two) {
+var mergeFast = function(one: Object, two: Object): Object {
   var ret = {};
   for (var keyOne in one) {
     ret[keyOne] = one[keyOne];
diff --git a/Libraries/Utilities/mergeIntoFast.js b/Libraries/Utilities/mergeIntoFast.js
index 666e7384e..1a97d07b7 100644
--- a/Libraries/Utilities/mergeIntoFast.js
+++ b/Libraries/Utilities/mergeIntoFast.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule mergeIntoFast
+ * @flow
  */
 'use strict';
 
@@ -17,7 +18,7 @@
  * @param {object} one Object to assign to.
  * @param {object} two Object to assign from.
  */
-var mergeIntoFast = function(one, two) {
+var mergeIntoFast = function(one: Object, two: Object): void {
   for (var keyTwo in two) {
     one[keyTwo] = two[keyTwo];
   }
diff --git a/Libraries/Utilities/nativeModulePrefixNormalizer.js b/Libraries/Utilities/nativeModulePrefixNormalizer.js
index 3c33fa503..151417bc5 100644
--- a/Libraries/Utilities/nativeModulePrefixNormalizer.js
+++ b/Libraries/Utilities/nativeModulePrefixNormalizer.js
@@ -7,11 +7,14 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule nativeModulePrefixNormalizer
+ * @flow
  */
 'use strict';
 
 // Dirty hack to support old (RK) and new (RCT) native module name conventions
-function nativeModulePrefixNormalizer(modules) {
+function nativeModulePrefixNormalizer(
+  modules: {[key: string]: any}
+): void {
   Object.keys(modules).forEach((moduleName) => {
     var strippedName = moduleName.replace(/^(RCT|RK)/, '');
     if (modules['RCT' + strippedName] && modules['RK' + strippedName]) {
diff --git a/Libraries/Utilities/truncate.js b/Libraries/Utilities/truncate.js
index b35c06588..1d318e835 100644
--- a/Libraries/Utilities/truncate.js
+++ b/Libraries/Utilities/truncate.js
@@ -7,11 +7,18 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule truncate
+ * @flow
  */
 'use strict';
 
 var merge = require('merge');
 
+type truncateOptions = {
+  breakOnWords: boolean;
+  minDelta: number;
+  elipsis: string;
+}
+
 var defaultOptions = {
   breakOnWords: true,
   minDelta: 10, // Prevents truncating a tiny bit off the end
@@ -19,7 +26,11 @@ var defaultOptions = {
 };
 
 // maxChars (including elipsis)
-var truncate = function(str, maxChars, options) {
+var truncate = function(
+  str: ?string,
+  maxChars: number,
+  options: truncateOptions
+): ?string {
   options = merge(defaultOptions, options);
   if (str && str.length &&
       str.length - options.minDelta + options.elipsis.length >= maxChars) {
diff --git a/Libraries/Vibration/VibrationIOS.ios.js b/Libraries/Vibration/VibrationIOS.ios.js
index c24a50606..2a1dc701c 100644
--- a/Libraries/Vibration/VibrationIOS.ios.js
+++ b/Libraries/Vibration/VibrationIOS.ios.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule VibrationIOS
+ * @flow
  */
 'use strict';