diff --git a/package.json b/package.json
index f5bd47622..03fa2a45d 100644
--- a/package.json
+++ b/package.json
@@ -20,7 +20,8 @@
     "prepack": "bin/prepack.js",
     "prepack-repl": "bin/prepack-repl.js"
   },
-  "main": "lib/prepack.js",
+  "main": "lib/prepack-node.js",
+  "browser": "lib/prepack-standalone.js",
   "scripts": {
     "build": "babel src --out-dir lib --source-maps && npm run build-bundle",
     "build-bundle": "webpack",
diff --git a/src/options.js b/src/options.js
new file mode 100644
index 000000000..016ceea3a
--- /dev/null
+++ b/src/options.js
@@ -0,0 +1,71 @@
+/**
+ * Copyright (c) 2017-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 */
+
+import type { RealmOptions } from "./types";
+import type { SerializerOptions } from "./serializer/types";
+
+export type Options = {
+  filename?: string,
+  inputSourceMapFilename?: string,
+  sourceMaps?: boolean,
+  compatibility?: "browser" | "jsc-600-1-4-17",
+  mathRandomSeed?: string,
+  speculate?: boolean,
+  trace?: boolean,
+  debugNames?: boolean,
+  singlePass?: boolean,
+  logStatistics?: boolean,
+  logModules?: boolean,
+  delayUnsupportedRequires?: boolean,
+  internalDebug?: boolean,
+  uniqueSuffix?: string,
+  timeout?: number,
+  strictlyMonotonicDateNow?: boolean,
+};
+
+export function getRealmOptions({
+  compatibility = "browser",
+  mathRandomSeed,
+  debugNames = false,
+  uniqueSuffix,
+  timeout,
+  strictlyMonotonicDateNow
+}: Options): RealmOptions {
+  return {
+    partial: true,
+    compatibility,
+    mathRandomSeed,
+    debugNames,
+    uniqueSuffix,
+    timeout,
+    strictlyMonotonicDateNow,
+  };
+}
+
+export function getSerializerOptions({
+  speculate = false,
+  trace = false,
+  singlePass = false,
+  logStatistics = false,
+  logModules = false,
+  delayUnsupportedRequires = false,
+  internalDebug = false
+}: Options): SerializerOptions {
+  return {
+    initializeMoreModules: speculate,
+    trace,
+    singlePass,
+    logStatistics,
+    logModules,
+    delayUnsupportedRequires,
+    internalDebug,
+  };
+}
diff --git a/src/prepack-cli.js b/src/prepack-cli.js
index 9b4e8b840..27a05b656 100644
--- a/src/prepack-cli.js
+++ b/src/prepack-cli.js
@@ -9,7 +9,8 @@
 
 /* @flow */
 
-import { run } from "./prepack.js";
+import { prepackFileSync, InitializationError } from "./prepack-node.js";
+import fs from "fs";
 
 let args = Array.from(process.argv);
 args.splice(0, 2);
@@ -17,8 +18,8 @@ let inputFilename;
 let outputFilename;
 let compatibility;
 let mathRandomSeed;
-let inputMap;
-let ouputMap;
+let inputSourceMap;
+let outputSourceMap;
 let flags = {
   speculate: false,
   trace: false,
@@ -27,6 +28,7 @@ let flags = {
   logStatistics: false,
   logModules: false,
   delayUnsupportedRequires: false,
+  internalDebug: false,
 };
 while (args.length) {
   let arg = args[0]; args.shift();
@@ -52,10 +54,10 @@ while (args.length) {
         mathRandomSeed = args[0]; args.shift();
         break;
       case "srcmapIn":
-        inputMap = args[0]; args.shift();
+        inputSourceMap = args[0]; args.shift();
         break;
       case "srcmapOut":
-        ouputMap = args[0]; args.shift();
+        outputSourceMap = args[0]; args.shift();
         break;
       case "help":
         console.log("Usage: prepack.js [ --out output.js ] [ --compatibility jsc ] [ --mathRandomSeed seedvalue ] [ --srcmapIn inputMap ] [ --srcmapOut outputMap ] [ --speculate ] [ --trace ] [ -- | input.js ] [ --singlePass ] [ --debugNames ]");
@@ -70,9 +72,44 @@ while (args.length) {
     }
   }
 }
+
 if (!inputFilename) {
   console.error("Missing input file.");
   process.exit(1);
 } else {
-  run(inputFilename, compatibility, mathRandomSeed, outputFilename, inputMap, ouputMap, flags.speculate, flags.trace, flags.debugNames, flags.singlePass, flags.logStatistics, flags.logModules, flags.delayUnsupportedRequires);
+  try {
+    let serialized = prepackFileSync(inputFilename, {
+      compatibility,
+      mathRandomSeed,
+      inputSourceMapFilename: inputSourceMap,
+      sourceMaps: !!outputSourceMap,
+      ...flags
+    });
+
+    let code = serialized.code;
+
+    if (code.length >= 1000 || outputFilename) {
+      let filename = outputFilename || (inputFilename + "-processed.js");
+      console.log(`Prepacked source code written to ${filename}.`);
+      fs.writeFileSync(filename, code);
+    }
+
+    if (code.length <= 1000 && !outputFilename) {
+      console.log("+++++++++++++++++ Prepacked source code");
+      console.log(code);
+      console.log("=================");
+    }
+
+    if (outputSourceMap) {
+      fs.writeFileSync(outputSourceMap, serialized.map || "");
+    }
+  } catch (x) {
+    if (x instanceof InitializationError) {
+      // Ignore InitializationError since they have already logged
+      // their errors to the console, but exit with an error code.
+      process.exit(1);
+    }
+    // For any other type of error, rethrow.
+    throw x;
+  }
 }
diff --git a/src/prepack-node.js b/src/prepack-node.js
new file mode 100644
index 000000000..791298641
--- /dev/null
+++ b/src/prepack-node.js
@@ -0,0 +1,67 @@
+/**
+ * Copyright (c) 2017-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 */
+import Serializer from "./serializer/index.js";
+import fs from "fs";
+import { getRealmOptions, getSerializerOptions } from "./options";
+import { InitializationError } from "./prepack-standalone";
+
+import type { Options } from "./options";
+
+export * from "./prepack-standalone";
+
+export function prepackFile(filename: string, options: Options = {}, callback: Function) {
+  let sourceMapFilename = options.inputSourceMapFilename || (filename + ".map");
+  fs.readFile(filename, "utf8", function(fileErr, code) {
+    if (fileErr) {
+      callback(fileErr);
+      return;
+    }
+    fs.readFile(sourceMapFilename, "utf8", function(mapErr, sourceMap) {
+      if (mapErr) {
+        console.log(`No sourcemap found at ${sourceMapFilename}.`);
+        sourceMap = "";
+      }
+      let serialized;
+      try {
+        serialized = new Serializer(
+          getRealmOptions(options),
+          getSerializerOptions(options),
+        ).init(filename, code, sourceMap, options.sourceMaps);
+        if (!serialized) {
+          throw new InitializationError();
+        }
+      } catch (err) {
+        callback(err);
+        return;
+      }
+      callback(null, serialized);
+    });
+  });
+}
+
+export function prepackFileSync(filename: string, options: Options = {}) {
+  let code = fs.readFileSync(filename, "utf8");
+  let sourceMap = "";
+  let sourceMapFilename = options.inputSourceMapFilename || (filename + ".map");
+  try {
+    sourceMap = fs.readFileSync(sourceMapFilename, "utf8");
+  } catch (_e) {
+    console.log(`No sourcemap found at ${sourceMapFilename}.`);
+  }
+  let serialized = new Serializer(
+    getRealmOptions(options),
+    getSerializerOptions(options),
+  ).init(filename, code, sourceMap, options.sourceMaps);
+  if (!serialized) {
+    throw new InitializationError();
+  }
+  return serialized;
+}
diff --git a/src/prepack-standalone.js b/src/prepack-standalone.js
new file mode 100644
index 000000000..3cb815a31
--- /dev/null
+++ b/src/prepack-standalone.js
@@ -0,0 +1,59 @@
+/**
+ * Copyright (c) 2017-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 */
+import Serializer from "./serializer/index.js";
+import * as t from "babel-types";
+import { getRealmOptions, getSerializerOptions } from "./options";
+
+import type { Options } from "./options";
+import type { BabelNodeFile, BabelNodeProgram } from "babel-types";
+
+// This should just be a class but Babel classes doesn't work with
+// built-in super classes.
+export function InitializationError() {
+  let self = new Error("An error occurred while prepacking. See the error logs.");
+  Object.setPrototypeOf(self, InitializationError.prototype);
+  return self;
+}
+Object.setPrototypeOf(InitializationError, Error);
+Object.setPrototypeOf(InitializationError.prototype, Error.prototype);
+
+
+export function prepack(code: string, options: Options = {}) {
+  let filename = options.filename || 'unknown';
+  let serialized = new Serializer(
+    getRealmOptions(options),
+    getSerializerOptions(options),
+  ).init(filename, code, "", false);
+  if (!serialized) {
+    throw new InitializationError();
+  }
+  return serialized;
+}
+
+export function prepackFromAst(ast: BabelNodeFile | BabelNodeProgram, code: string, options: Options = {}) {
+  if (ast && ast.type === "Program") {
+    ast = t.file(ast, [], []);
+  } else if (!ast || ast.type !== "File") {
+    throw new Error("Not a valid ast?");
+  }
+
+  // TODO: Expose an option to wire an already parsed ast all the way through
+  // to the execution environment. For now, we just reparse.
+
+  let serialized = new Serializer(
+    getRealmOptions(options),
+    getSerializerOptions(options),
+  ).init("", code, "", options.sourceMaps);
+  if (!serialized) {
+    throw new InitializationError();
+  }
+  return serialized;
+}
diff --git a/src/prepack.js b/src/prepack.js
deleted file mode 100644
index f3c4269b5..000000000
--- a/src/prepack.js
+++ /dev/null
@@ -1,81 +0,0 @@
-/**
- * Copyright (c) 2017-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 */
-import Serializer from "./serializer/index.js";
-import invariant from "./invariant.js";
-let fs        = require("fs");
-
-function run_internal(
-    name: string,
-    raw: string,
-    map: string = "",
-    compatibility?: "browser" | "jsc-600-1-4-17" = "browser",
-    mathRandomSeed: void | string,
-    outputFilename?: string,
-    outputMap?: string,
-    speculate: boolean = false,
-    trace: boolean = false,
-    debugNames: boolean = false,
-    singlePass: boolean = false,
-    logStatistics: boolean = false,
-    logModules: boolean = false,
-    delayUnsupportedRequires: boolean = false) {
-  let serialized =
-    new Serializer(
-      { partial: true, compatibility, mathRandomSeed, debugNames },
-      { initializeMoreModules: speculate, internalDebug: true, trace, singlePass, logStatistics, logModules, delayUnsupportedRequires })
-        .init(name, raw, map, outputMap !== undefined);
-  if (!serialized) {
-    process.exit(1);
-    invariant(false);
-  }
-  let code = serialized.code;
-
-  if (code.length >= 1000 || outputFilename) {
-    let filename = outputFilename || (name + "-processed.js");
-    console.log(`Prepacked source code written to ${filename}.`);
-    fs.writeFileSync(filename, code);
-  }
-
-  if (code.length <= 1000 && !outputFilename) {
-    console.log("+++++++++++++++++ Prepacked source code");
-    console.log(code);
-    console.log("=================");
-  }
-
-  if (outputMap) {
-    fs.writeFileSync(outputMap, map);
-  }
-}
-
-export function run(
-    inFn: string,
-    compat?: "browser" | "jsc-600-1-4-17" = "browser",
-    mathRandSeed: void | string,
-    outFn?: string,
-    inputMap?: string,
-    outMap?: string,
-    speculateOpt?: boolean,
-    trace?: boolean,
-    debugNames?: boolean,
-    singlePass?: boolean,
-    logStatistics?: boolean,
-    logModules?: boolean,
-    delayUnsupportedRequires?: boolean) {
-  let input = fs.readFileSync(inFn, "utf8");
-  let map = "";
-  let mapFile = inputMap ? inputMap : inFn + ".map";
-  try {
-    map = fs.readFileSync(mapFile, "utf8");
-  } catch (_e) {
-    console.log(`No sourcemap found at ${mapFile}.`);
-  }
-  run_internal(inFn, input, map, compat, mathRandSeed, outFn, outMap, speculateOpt, trace, debugNames, singlePass, logStatistics, logModules, delayUnsupportedRequires);
-}
diff --git a/webpack.config.js b/webpack.config.js
index be7377182..d004f955a 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -11,11 +11,11 @@ const path = require('path');
 const webpack = require('webpack');
 
 const WebpackConfig = {
-    entry: "./lib/serializer/index.js",
+    entry: "./",
     output: {
         path: path.join(__dirname),
         filename: "prepack.min.js",
-        library: 'prepack',
+        library: 'Prepack',
     },
 
     plugins: [