diff --git a/chromite/bin/cros_build_packages b/chromite/bin/cros_build_packages
new file mode 100755
index 0000000000..0cf2f4f301
--- /dev/null
+++ b/chromite/bin/cros_build_packages
@@ -0,0 +1,275 @@
+#!/usr/bin/python2.6
+# Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import optparse
+import os
+import multiprocessing
+import sys
+import tempfile
+sys.path.insert(0, os.path.abspath(__file__ + "/../../lib"))
+from cros_build_lib import Die
+from cros_build_lib import Info
+from cros_build_lib import RunCommand
+from cros_build_lib import Warning
+
+
+def BuildPackages():
+  """Build packages according to options specified on command-line."""
+
+  if os.getuid() != 0:
+    Die("superuser access required")
+
+  scripts_dir = os.path.abspath(__file__ + "/../../..")
+  builder = PackageBuilder(scripts_dir)
+  options, _ = builder.ParseArgs()
+
+  # Calculate packages to install.
+  # TODO(davidjames): Grab these from a spec file.
+  packages = ["chromeos-base/chromeos"]
+  if options.withdev:
+    packages.append("chromeos-base/chromeos-dev")
+  if options.withfactory:
+    packages.append("chromeos-base/chromeos-factoryinstall")
+  if options.withtest:
+    packages.append("chromeos-base/chromeos-test")
+
+  if options.usetarball:
+    builder.ExtractTarball(options, packages)
+  else:
+    builder.BuildTarball(options, packages)
+
+
+def _Apply(args):
+  """Call the function specified in args[0], with arguments in args[1:]."""
+  return apply(args[0], args[1:])
+
+
+def _GetLatestPrebuiltPrefix(board):
+  """Get the latest prebuilt prefix for the specified board.
+
+  Args:
+    board: The board you want prebuilts for.
+  Returns:
+    Latest prebuilt prefix.
+  """
+  # TODO(davidjames): Also append profile names here.
+  prefix = "http://commondatastorage.googleapis.com/chromeos-prebuilt/board"
+  tmpfile = tempfile.NamedTemporaryFile()
+  _Run("curl '%s/%s-latest' -o %s" % (prefix, board, tmpfile.name), retries=3)
+  tmpfile.seek(0)
+  latest = tmpfile.read().strip()
+  tmpfile.close()
+  return "%s/%s" % (prefix, latest)
+
+
+def _GetPrebuiltDownloadCommands(prefix):
+  """Return a list of commands for grabbing packages.
+
+  There must be a file called "packages/Packages" that contains the list of
+  packages. The specified list of commands will fill the packages directory
+  with the bzipped packages from the specified prefix.
+
+  Args:
+    prefix: Url prefix to download packages from.
+  Returns:
+    List of commands for grabbing packages.
+  """
+
+  cmds = []
+  for line in file("packages/Packages"):
+    if line.startswith("CPV: "):
+      pkgpath, pkgname = line.replace("CPV: ", "").strip().split("/")
+      path = "%s/%s.tbz2" % (pkgpath, pkgname)
+      url = "%s/%s" % (prefix, path)
+      dirname = "packages/%s" % pkgpath
+      fullpath = "packages/%s" % path
+      if not os.path.exists(dirname):
+        os.makedirs(dirname)
+      if not os.path.exists(fullpath):
+        cmds.append("curl -s %s -o %s" % (url, fullpath))
+  return cmds
+
+
+def _Run(cmd, retries=0):
+  """Run the specified command.
+
+  If the command fails, and the retries have been exhausted, the program exits
+  with an appropriate error message.
+
+  Args:
+    cmd: The command to run.
+    retries: If exit code is non-zero, retry this many times.
+  """
+  # TODO(davidjames): Move this to common library.
+  for _ in range(retries+1):
+    result = RunCommand(cmd, shell=True, exit_code=True, error_ok=True)
+    if result.returncode == 0:
+      Info("Command succeeded: %s" % cmd)
+      break
+    Warning("Command failed: %s" % cmd)
+  else:
+    Die("Command failed, exiting: %s" % cmd)
+
+
+def _RunManyParallel(cmds, retries=0):
+  """Run list of provided commands in parallel.
+
+  To work around a bug in the multiprocessing module, we use map_async instead
+  of the usual map function. See http://bugs.python.org/issue9205
+
+  Args:
+    cmds: List of commands to run.
+    retries: Number of retries per command.
+  """
+  # TODO(davidjames): Move this to common library.
+  pool = multiprocessing.Pool()
+  args = []
+  for cmd in cmds:
+    args.append((_Run, cmd, retries))
+  result = pool.map_async(_Apply, args, chunksize=1)
+  while True:
+    try:
+      result.get(60*60)
+      break
+    except multiprocessing.TimeoutError:
+      pass
+
+
+class PackageBuilder(object):
+  """A class for building and extracting tarballs of Chromium OS packages."""
+
+  def __init__(self, scripts_dir):
+    self.scripts_dir = scripts_dir
+
+  def BuildTarball(self, options, packages):
+    """Build a tarball with the specified packages.
+
+    Args:
+      options: Options object, as output by ParseArgs.
+      packages: List of packages to build.
+    """
+
+    board = options.board
+
+    # Run setup_board. TODO(davidjames): Integrate the logic used in
+    # setup_board into chromite.
+    _Run("%s/setup_board --force --board=%s" % (self.scripts_dir, board))
+
+    # Create complete build directory
+    _Run(self._EmergeBoardCmd(options, packages))
+
+    # Archive build directory as tarballs
+    os.chdir("/build/%s" % board)
+    cmds = [
+        "tar -c --wildcards --exclude='usr/lib/debug/*' "
+        "--exclude='packages/*' * | pigz -c > packages/%s-build.tgz" % board,
+        "tar -c usr/lib/debug/* | pigz -c > packages/%s-debug.tgz" % board
+    ]
+
+    # Run list of commands.
+    _RunManyParallel(cmds)
+
+  def ExtractTarball(self, options, packages):
+    """Extract the latest build tarball, then update the specified packages.
+
+    Args:
+      options: Options object, as output by ParseArgs.
+      packages: List of packages to update.
+    """
+
+    board = options.board
+    prefix = _GetLatestPrebuiltPrefix(board)
+
+    # If the user doesn't have emerge-${BOARD} setup yet, we need to run
+    # setup_board. TODO(davidjames): Integrate the logic used in setup_board
+    # into chromite.
+    if not os.path.exists("/usr/local/bin/emerge-%s" % board):
+      _Run("%s/setup_board --force --board=%s" % (self.scripts_dir, board))
+
+    # Delete old build directory. This process might take a while, so do it in
+    # the background.
+    cmds = []
+    if os.path.exists("/build/%s" % board):
+      tempdir = tempfile.mkdtemp()
+      _Run("mv /build/%s %s" % (board, tempdir))
+      cmds.append("rm -rf %s" % tempdir)
+
+    # Create empty build directory, and chdir into it.
+    os.makedirs("/build/%s/packages" % board)
+    os.chdir("/build/%s" % board)
+
+    # Download and expand build tarball.
+    build_url = "%s/%s-build.tgz" % (prefix, board)
+    cmds.append("curl -s %s | tar -xz" % build_url)
+
+    # Download and expand debug tarball (if requested).
+    if options.debug:
+      debug_url = "%s/%s-debug.tgz" % (prefix, board)
+      cmds.append("curl -s %s | tar -xz" % debug_url)
+
+    # Download prebuilt packages.
+    _Run("curl '%s/Packages' -o packages/Packages" % prefix, retries=3)
+    cmds.extend(_GetPrebuiltDownloadCommands(prefix))
+
+    # Run list of commands, with three retries per command, in case the network
+    # is flaky.
+    _RunManyParallel(cmds, retries=3)
+
+    # Emerge remaining packages.
+    _Run(self._EmergeBoardCmd(options, packages))
+
+  def ParseArgs(self):
+    """Parse arguments from the command line using optparse."""
+
+    # TODO(davidjames): We should use spec files for this.
+    default_board = self._GetDefaultBoard()
+    parser = optparse.OptionParser()
+    parser.add_option("--board", dest="board", default=default_board,
+                      help="The board to build packages for.")
+    parser.add_option("--debug", action="store_true", dest="debug",
+                      default=False, help="Include debug symbols.")
+    parser.add_option("--nowithdev", action="store_false", dest="withdev",
+                      default=True,
+                      help="Don't build useful developer friendly utilities.")
+    parser.add_option("--nowithtest", action="store_false", dest="withtest",
+                      default=True, help="Build packages required for testing.")
+    parser.add_option("--nowithfactory", action="store_false",
+                      dest="withfactory", default=True,
+                      help="Build factory installer")
+    parser.add_option("--nousepkg", action="store_false",
+                      dest="usepkg", default=True,
+                      help="Don't use binary packages.")
+    parser.add_option("--nousetarball", action="store_false",
+                      dest="usetarball", default=True,
+                      help="Don't use tarball.")
+    parser.add_option("--nofast", action="store_false", dest="fast",
+                      default=True,
+                      help="Don't merge packages in parallel.")
+
+    return parser.parse_args()
+
+  def _EmergeBoardCmd(self, options, packages):
+    """Calculate board emerge command."""
+    board = options.board
+    scripts_dir = self.scripts_dir
+    emerge_board = "emerge-%s" % board
+    if options.fast:
+      emerge_board = "%s/parallel_emerge --board=%s" % (scripts_dir, board)
+    usepkg = ""
+    if options.usepkg:
+      usepkg = "g"
+    return "%s -uDNv%s %s" % (emerge_board, usepkg, " ".join(packages))
+
+  def _GetDefaultBoard(self):
+    """Get the default board configured by the user."""
+
+    default_board_file = "%s/.default_board" % self.scripts_dir
+    default_board = None
+    if os.path.exists(default_board_file):
+      default_board = file(default_board_file).read().strip()
+    return default_board
+
+if __name__ == "__main__":
+  BuildPackages()