diff --git a/packages/metro/src/commands/serve.js b/packages/metro/src/commands/serve.js
index 6d056796..1f274f97 100644
--- a/packages/metro/src/commands/serve.js
+++ b/packages/metro/src/commands/serve.js
@@ -48,7 +48,7 @@ exports.builder = (yargs: Yargs) => {
   yargs.option('secure-key', {type: 'string'});
   yargs.option('secure-cert', {type: 'string'});
 
-  yargs.option('legacy-bundler', {type: 'boolean'});
+  yargs.option('hmr-enabled', {alias: 'hmr', type: 'boolean'});
 
   yargs.option('config', {alias: 'c', type: 'string'});
 
diff --git a/packages/metro/src/index.js b/packages/metro/src/index.js
index 6df0b7f2..f6f84dea 100644
--- a/packages/metro/src/index.js
+++ b/packages/metro/src/index.js
@@ -16,10 +16,12 @@ const Config = require('./Config');
 const Http = require('http');
 const Https = require('https');
 const MetroBundler = require('./shared/output/bundle');
+const MetroHmrServer = require('./HmrServer');
 const MetroServer = require('./Server');
 const TerminalReporter = require('./lib/TerminalReporter');
 const TransformCaching = require('./lib/TransformCaching');
 
+const attachWebsocketServer = require('./lib/attachWebsocketServer');
 const defaults = require('./defaults');
 
 const {realpath} = require('fs');
@@ -27,6 +29,7 @@ const {readFile} = require('fs-extra');
 const {Terminal} = require('metro-core');
 
 import type {ConfigT} from './Config';
+import type {Reporter} from './lib/reporting';
 import type {RequestOptions, OutputOptions} from './shared/types.flow.js';
 import type {Options as ServerOptions} from './shared/types.flow';
 import type {IncomingMessage, ServerResponse} from 'http';
@@ -40,6 +43,7 @@ type PublicMetroOptions = {|
   maxWorkers?: number,
   port?: ?number,
   projectRoots: Array<string>,
+  reporter?: Reporter,
   // deprecated
   resetCache?: boolean,
 |};
@@ -71,10 +75,9 @@ async function runMetro({
   // $FlowFixMe TODO t0 https://github.com/facebook/flow/issues/183
   port = null,
   projectRoots = [],
+  reporter = new TerminalReporter(new Terminal(process.stdout)),
   watch = false,
 }: PrivateMetroOptions): Promise<MetroServer> {
-  const reporter = new TerminalReporter(new Terminal(process.stdout));
-
   const normalizedConfig = config ? Config.normalize(config) : Config.DEFAULT;
 
   const assetExts = defaults.assetExts.concat(
@@ -163,6 +166,7 @@ exports.createConnectMiddleware = async function(
     : Config.DEFAULT;
 
   return {
+    metroServer,
     middleware: normalizedConfig.enhanceMiddleware(metroServer.processRequest),
     end() {
       metroServer.end();
@@ -178,21 +182,25 @@ type RunServerOptions = {|
   secure?: boolean,
   secureKey?: string,
   secureCert?: string,
+  hmrEnabled?: boolean,
 |};
 
 exports.runServer = async (options: RunServerOptions) => {
   const port = options.port || 8080;
+  const reporter =
+    options.reporter || new TerminalReporter(new Terminal(process.stdout));
 
   // Lazy require
   const connect = require('connect');
 
   const serverApp = connect();
 
-  const {middleware, end} = await exports.createConnectMiddleware({
+  const {metroServer, middleware, end} = await exports.createConnectMiddleware({
     config: options.config,
     maxWorkers: options.maxWorkers,
     port,
     projectRoots: options.projectRoots,
+    reporter,
     resetCache: options.resetCache,
   });
 
@@ -212,6 +220,14 @@ exports.runServer = async (options: RunServerOptions) => {
     httpServer = Http.createServer(serverApp);
   }
 
+  if (options.hmrEnabled) {
+    attachWebsocketServer({
+      httpServer,
+      path: '/hot',
+      websocketServer: new MetroHmrServer(metroServer, reporter),
+    });
+  }
+
   httpServer.listen(port, options.host, () => {
     options.onReady && options.onReady(httpServer);
   });
diff --git a/packages/metro/src/lib/attachWebsocketServer.js b/packages/metro/src/lib/attachWebsocketServer.js
new file mode 100644
index 00000000..8cbbdffa
--- /dev/null
+++ b/packages/metro/src/lib/attachWebsocketServer.js
@@ -0,0 +1,83 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ * @format
+ */
+
+'use strict';
+
+import type {Server as HttpServer} from 'http';
+import type {Server as HttpsServer} from 'https';
+
+type WebsocketServiceInterface<T> = {
+  +onClientConnect: (
+    url: string,
+    sendFn: (data: string) => mixed,
+  ) => Promise<T>,
+  +onClientDisconnect?: (client: T) => mixed,
+  +onClientError?: (client: T, e: Error) => mixed,
+  +onClientMessage?: (client: T, message: string) => mixed,
+};
+
+type HMROptions<TClient> = {
+  httpServer: HttpServer | HttpsServer,
+  websocketServer: WebsocketServiceInterface<TClient>,
+  path: string,
+};
+
+/**
+ * Attach a websocket server to an already existing HTTP[S] server, and forward
+ * the received events on the given "websocketServer" parameter. It must be an
+ * object with the following fields:
+ *
+ *   - onClientConnect
+ *   - onClientError
+ *   - onClientMessage
+ *   - onClientDisconnect
+ */
+
+module.exports = function attachWebsocketServer<TClient: Object>({
+  httpServer,
+  websocketServer,
+  path,
+}: HMROptions<TClient>) {
+  const WebSocketServer = require('ws').Server;
+  const wss = new WebSocketServer({
+    server: httpServer,
+    path,
+  });
+
+  wss.on('connection', async ws => {
+    let connected = true;
+    const url = ws.upgradeReq.url;
+
+    const sendFn = (...args) => {
+      if (connected) {
+        ws.send(...args);
+      }
+    };
+
+    const client = await websocketServer.onClientConnect(url, sendFn);
+
+    ws.on('error', e => {
+      websocketServer.onClientError && websocketServer.onClientError(client, e);
+    });
+
+    ws.on('close', () => {
+      websocketServer.onClientDisconnect &&
+        websocketServer.onClientDisconnect(client);
+      connected = false;
+    });
+
+    ws.on('message', message => {
+      websocketServer.onClientMessage &&
+        websocketServer.onClientMessage(client, message);
+    });
+  });
+};