diff --git a/.eslintrc b/.eslintrc
index 5cf245ac59..90e3903312 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -10,6 +10,10 @@
     "react/jsx-uses-vars": 2,
     "react/react-in-jsx-scope": 2,
 
+    // Disable until Flow supports let and const
+    "no-var": 0,
+    "vars-on-top": 0,
+
     //Temporarirly disabled due to a possible bug in babel-eslint (todomvc example)
     "block-scoped-var": 0,
     // Temporarily disabled for test/* until babel/babel-eslint#33 is resolved
diff --git a/.flowconfig b/.flowconfig
new file mode 100644
index 0000000000..369768b7ad
--- /dev/null
+++ b/.flowconfig
@@ -0,0 +1,9 @@
+[ignore]
+.*/lib
+.*/test
+
+[include]
+
+[libs]
+
+[options]
diff --git a/src/createStore.js b/src/createStore.js
index 78ab81f1d7..9e116a1ae9 100644
--- a/src/createStore.js
+++ b/src/createStore.js
@@ -1,37 +1,45 @@
+/* @flow */
+/*eslint-disable */
+import type { State, Reducer, Action, IntermediateAction, Store } from './types';
+/*eslint-enable */
+
 import invariant from 'invariant';
 import isPlainObject from './utils/isPlainObject';
 
 // Don't ever try to handle these action types in your code. They are private.
 // For any unknown actions, you must return the current state.
 // If the current state is undefined, you must return the initial state.
-export const ActionTypes = {
+export var ActionTypes = {
   INIT: '@@redux/INIT'
 };
 
-export default function createStore(reducer, initialState) {
+export default function createStore(
+  reducer: Reducer,
+  initialState: State
+): Store {
   invariant(
     typeof reducer === 'function',
     'Expected the reducer to be a function.'
   );
 
-  let currentReducer = null;
-  let currentState = initialState;
-  let listeners = [];
+  var currentReducer = reducer;
+  var currentState = initialState;
+  var listeners = [];
 
   function getState() {
     return currentState;
   }
 
-  function subscribe(listener) {
+  function subscribe(listener: Function) {
     listeners.push(listener);
 
     return function unsubscribe() {
-      const index = listeners.indexOf(listener);
+      var index = listeners.indexOf(listener);
       listeners.splice(index, 1);
     };
   }
 
-  function dispatch(action) {
+  function dispatch(action: Action) {
     invariant(
       isPlainObject(action),
       'Actions must be plain objects. Use custom middleware for async actions.'
@@ -46,12 +54,12 @@ export default function createStore(reducer, initialState) {
     return currentReducer;
   }
 
-  function replaceReducer(nextReducer) {
+  function replaceReducer(nextReducer: Reducer) {
     currentReducer = nextReducer;
     dispatch({ type: ActionTypes.INIT });
   }
 
-  replaceReducer(reducer);
+  dispatch({ type: ActionTypes.INIT });
 
   return {
     dispatch,
diff --git a/src/index.js b/src/index.js
index 1cfbbf6489..dd0cdd5f11 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,7 +1,7 @@
-// Core
+/* @flow */
+
 import createStore from './createStore';
 
-// Utilities
 import compose from './utils/compose';
 import combineReducers from './utils/combineReducers';
 import bindActionCreators from './utils/bindActionCreators';
diff --git a/src/types.js b/src/types.js
new file mode 100644
index 0000000000..740c1b039f
--- /dev/null
+++ b/src/types.js
@@ -0,0 +1,40 @@
+/* @flow */
+
+export type State = any;
+
+export type Action = Object;
+
+export type IntermediateAction = any;
+
+export type Dispatch = (a: Action | IntermediateAction) => any;
+
+export type Reducer<S, A> = (state: S, action: A) => S;
+
+export type ActionCreator = (...args: any) =>
+  Action | IntermediateAction;
+
+export type MiddlewareArgs = {
+  dispatch: Dispatch;
+  getState: () => State;
+};
+
+export type Middleware = (args: MiddlewareArgs) =>
+  (next: Dispatch) =>
+  Dispatch;
+
+export type Store = {
+  dispatch: Dispatch;
+  getState: () => State;
+  getReducer: Reducer;
+  replaceReducer: (nextReducer: Reducer) => void;
+  subscribe: (listener: () => void) => () => void;
+};
+
+export type CreateStore = (
+  reducer: Reducer,
+  initialState: State
+) => Store;
+
+export type HigherOrderStore = (
+  next: CreateStore
+) => CreateStore;
diff --git a/src/utils/applyMiddleware.js b/src/utils/applyMiddleware.js
index ac1434260a..00140faf27 100644
--- a/src/utils/applyMiddleware.js
+++ b/src/utils/applyMiddleware.js
@@ -1,3 +1,11 @@
+/* @flow */
+/*eslint-disable */
+import type {
+  Dispatch, Middleware, Reducer, State,
+  Store, CreateStore, HigherOrderStore
+} from '../types';
+/*eslint-enable */
+
 import compose from './compose';
 import composeMiddleware from './composeMiddleware';
 
@@ -8,23 +16,27 @@ import composeMiddleware from './composeMiddleware';
  * @param {...Function} ...middlewares
  * @return {Function} A higher-order store
  */
-export default function applyMiddleware(...middlewares) {
-  return next => (...args) => {
-    const store = next(...args);
-    const middleware = composeMiddleware(...middlewares);
-
-    let composedDispatch = null;
+export default function applyMiddleware(
+  ...middlewares: Array<Middleware>
+): HigherOrderStore {
+  return (next: CreateStore) => (reducer: Reducer, initialState: State) => {
+    var store = next(reducer, initialState);
+    var middleware = composeMiddleware(...middlewares);
+    var composedDispatch = () => {};
 
     function dispatch(action) {
       return composedDispatch(action);
     }
 
-    const methods = {
-      dispatch,
-      getState: store.getState
+    var middlewareAPI = {
+      getState: store.getState,
+      dispatch
     };
 
-    composedDispatch = compose(middleware(methods), store.dispatch);
+    composedDispatch = compose(
+      middleware(middlewareAPI),
+      store.dispatch
+    );
 
     return {
       ...store,
diff --git a/src/utils/bindActionCreators.js b/src/utils/bindActionCreators.js
index f9a81a5a80..0775b9d27a 100644
--- a/src/utils/bindActionCreators.js
+++ b/src/utils/bindActionCreators.js
@@ -1,6 +1,14 @@
+/* @flow */
+/*eslint-disable */
+import type { Dispatch } from '../types';
+/*eslint-enable */
+
 import mapValues from '../utils/mapValues';
 
-export default function bindActionCreators(actionCreators, dispatch) {
+export default function bindActionCreators(
+  actionCreators: Object,
+  dispatch: Dispatch
+): Object {
   return mapValues(actionCreators, actionCreator =>
     (...args) => dispatch(actionCreator(...args))
   );
diff --git a/src/utils/combineReducers.js b/src/utils/combineReducers.js
index dbf6c966ec..54a74a4c82 100644
--- a/src/utils/combineReducers.js
+++ b/src/utils/combineReducers.js
@@ -1,11 +1,16 @@
+/* @flow */
+/*eslint-disable */
+import type { Action, State, Reducer } from '../types';
+/*eslint-enable */
+
 import mapValues from '../utils/mapValues';
 import pick from '../utils/pick';
 import invariant from 'invariant';
 import { ActionTypes } from '../createStore';
 
-function getErrorMessage(key, action) {
-  const actionType = action && action.type;
-  const actionName = actionType && `"${actionType}"` || 'an action';
+function getErrorMessage(key: String, action: Action): string {
+  var actionType = action && action.type;
+  var actionName = actionType && `"${actionType}"` || 'an action';
 
   return (
     `Reducer "${key}" returned undefined handling ${actionName}. ` +
@@ -13,11 +18,11 @@ function getErrorMessage(key, action) {
   );
 }
 
-export default function combineReducers(reducers) {
-  const finalReducers = pick(reducers, (val) => typeof val === 'function');
+export default function combineReducers(reducers: Object): Reducer {
+  var finalReducers = pick(reducers, (val) => typeof val === 'function');
 
   Object.keys(finalReducers).forEach(key => {
-    const reducer = finalReducers[key];
+    var reducer = finalReducers[key];
     invariant(
       typeof reducer(undefined, { type: ActionTypes.INIT }) !== 'undefined',
       `Reducer "${key}" returned undefined during initialization. ` +
@@ -26,7 +31,7 @@ export default function combineReducers(reducers) {
       `not be undefined.`
     );
 
-    const type = Math.random().toString(36).substring(7).split('').join('.');
+    var type = Math.random().toString(36).substring(7).split('').join('.');
     invariant(
       typeof reducer(undefined, { type }) !== 'undefined',
       `Reducer "${key}" returned undefined when probed with a random type. ` +
@@ -38,9 +43,9 @@ export default function combineReducers(reducers) {
     );
   });
 
-  return function composition(state = {}, action) {
+  return function composition(state: State = {}, action: Action): State {
     return mapValues(finalReducers, (reducer, key) => {
-      const newState = reducer(state[key], action);
+      var newState = reducer(state[key], action);
       invariant(
         typeof newState !== 'undefined',
         getErrorMessage(key, action)
diff --git a/src/utils/compose.js b/src/utils/compose.js
index 4db0884d3b..b4b291e015 100644
--- a/src/utils/compose.js
+++ b/src/utils/compose.js
@@ -1,8 +1,10 @@
+/* @flow */
+
 /**
  * Composes functions from left to right
  * @param  {...Function} funcs - Functions to compose
  * @return {Function}
  */
-export default function compose(...funcs) {
+export default function compose(...funcs: Array<Function>): Function {
   return funcs.reduceRight((composed, f) => f(composed));
 }
diff --git a/src/utils/composeMiddleware.js b/src/utils/composeMiddleware.js
index bf702aae30..8eee92b28c 100644
--- a/src/utils/composeMiddleware.js
+++ b/src/utils/composeMiddleware.js
@@ -1,3 +1,8 @@
+/* @flow */
+/*eslint-disable */
+import type { Dispatch, Middleware, MiddlewareArgs } from '../types';
+/*eslint-enable */
+
 import compose from './compose';
 
 /**
@@ -5,6 +10,12 @@ import compose from './compose';
  * @param  {...Function} middlewares
  * @return {Function}
  */
-export default function composeMiddleware(...middlewares) {
-  return methods => next => compose(...middlewares.map(m => m(methods)), next);
+export default function composeMiddleware(
+  ...middlewares: Array<Middleware>
+): Middleware {
+  return (args: MiddlewareArgs) => (next: Dispatch) => {
+    var dispatchChain = middlewares.map(middleware => middleware(args));
+    dispatchChain.push(next);
+    return compose.apply(null, dispatchChain);
+  };
 }
diff --git a/src/utils/isPlainObject.js b/src/utils/isPlainObject.js
index a5845486cf..b352d71181 100644
--- a/src/utils/isPlainObject.js
+++ b/src/utils/isPlainObject.js
@@ -1,3 +1,10 @@
-export default function isPlainObject(obj) {
-  return obj ? typeof obj === 'object' && Object.getPrototypeOf(obj) === Object.prototype : false;
+/* @flow */
+
+export default function isPlainObject(obj: Object): boolean {
+  if (!obj) {
+    return false;
+  }
+
+  return typeof obj === 'object' &&
+         Object.getPrototypeOf(obj) === Object.prototype;
 }
diff --git a/src/utils/mapValues.js b/src/utils/mapValues.js
index 29d203cf61..9e09aeaab8 100644
--- a/src/utils/mapValues.js
+++ b/src/utils/mapValues.js
@@ -1,4 +1,6 @@
-export default function mapValues(obj, fn) {
+/* @flow */
+
+export default function mapValues(obj: Object, fn: Function): Object {
   return Object.keys(obj).reduce((result, key) => {
     result[key] = fn(obj[key], key);
     return result;
diff --git a/src/utils/pick.js b/src/utils/pick.js
index 2c9719c1c0..7025cb35e3 100644
--- a/src/utils/pick.js
+++ b/src/utils/pick.js
@@ -1,4 +1,6 @@
-export default function pick(obj, fn) {
+/* @flow */
+
+export default function pick(obj: Object, fn: Function): Object {
   return Object.keys(obj).reduce((result, key) => {
     if (fn(obj[key])) {
       result[key] = obj[key];
diff --git a/test/utils/applyMiddleware.spec.js b/test/utils/applyMiddleware.spec.js
index d3f8f60ea5..0ca73d84c0 100644
--- a/test/utils/applyMiddleware.spec.js
+++ b/test/utils/applyMiddleware.spec.js
@@ -22,8 +22,8 @@ describe('applyMiddleware', () => {
     expect(spy.calls.length).toEqual(1);
 
     expect(Object.keys(spy.calls[0].arguments[0])).toEqual([
-      'dispatch',
-      'getState'
+      'getState',
+      'dispatch'
     ]);
 
     expect(store.getState()).toEqual([ { id: 1, text: 'Use Redux' }, { id: 2, text: 'Flux FTW!' } ]);