diff --git a/subcmds/sync.py b/subcmds/sync.py
new file mode 100644
index 0000000..3eb44ed
--- /dev/null
+++ b/subcmds/sync.py
@@ -0,0 +1,150 @@
+#
+# Copyright (C) 2008 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+import re
+import subprocess
+import sys
+
+from git_command import GIT
+from command import Command
+from error import RepoChangedException, GitError
+from project import R_HEADS
+
+class Sync(Command):
+  common = True
+  helpSummary = "Update working tree to the latest revision"
+  helpUsage = """
+%prog [<project>...]
+"""
+  helpDescription = """
+The '%prog' command synchronizes local project directories
+with the remote repositories specified in the manifest.  If a local
+project does not yet exist, it will clone a new local directory from
+the remote repository and set up tracking branches as specified in
+the manifest.  If the local project already exists, '%prog'
+will update the remote branches and rebase any new local changes
+on top of the new remote changes.
+
+'%prog' will synchronize all projects listed at the command
+line.  Projects can be specified either by name, or by a relative
+or absolute path to the project's local directory. If no projects
+are specified, '%prog' will synchronize all projects listed in
+the manifest.
+"""
+
+  def _Options(self, p):
+    p.add_option('--no-repo-verify',
+                 dest='no_repo_verify', action='store_true',
+                 help='do not verify repo source code')
+
+  def _Fetch(self, *projects):
+    fetched = set()
+    for project in projects:
+      if project.Sync_NetworkHalf():
+        fetched.add(project.gitdir)
+      else:
+        print >>sys.stderr, 'error: Cannot fetch %s' % project.name
+        sys.exit(1)
+    return fetched
+
+  def Execute(self, opt, args):
+    rp = self.manifest.repoProject
+    rp.PreSync()
+
+    mp = self.manifest.manifestProject
+    mp.PreSync()
+
+    all = self.GetProjects(args, missing_ok=True)
+    fetched = self._Fetch(rp, mp, *all)
+
+    if rp.HasChanges:
+      print >>sys.stderr, 'info: A new version of repo is available'
+      print >>sys.stderr, ''
+      if opt.no_repo_verify or _VerifyTag(rp):
+        if not rp.Sync_LocalHalf():
+          sys.exit(1)
+        print >>sys.stderr, 'info: Restarting repo with latest version'
+        raise RepoChangedException()
+      else:
+        print >>sys.stderr, 'warning: Skipped upgrade to unverified version'
+
+    if mp.HasChanges:
+      if not mp.Sync_LocalHalf():
+        sys.exit(1)
+
+      self.manifest._Unload()
+      all = self.GetProjects(args, missing_ok=True)
+      missing = []
+      for project in all:
+        if project.gitdir not in fetched:
+          missing.append(project)
+      self._Fetch(*missing)
+
+    for project in all:
+      if not project.Sync_LocalHalf():
+        sys.exit(1)
+
+
+def _VerifyTag(project):
+  gpg_dir = os.path.expanduser('~/.repoconfig/gnupg')
+  if not os.path.exists(gpg_dir):
+    print >>sys.stderr,\
+"""warning: GnuPG was not available during last "repo init"
+warning: Cannot automatically authenticate repo."""
+    return True
+
+  remote = project.GetRemote(project.remote.name)
+  ref = remote.ToLocal(project.revision)
+
+  try:
+    cur = project.bare_git.describe(ref)
+  except GitError:
+    cur = None
+
+  if not cur \
+     or re.compile(r'^.*-[0-9]{1,}-g[0-9a-f]{1,}$').match(cur):
+    rev = project.revision
+    if rev.startswith(R_HEADS):
+      rev = rev[len(R_HEADS):]
+
+    print >>sys.stderr
+    print >>sys.stderr,\
+      "warning: project '%s' branch '%s' is not signed" \
+      % (project.name, rev)
+    return False
+
+  env = dict(os.environ)
+  env['GIT_DIR'] = project.gitdir
+  env['GNUPGHOME'] = gpg_dir
+
+  cmd = [GIT, 'tag', '-v', cur]
+  proc = subprocess.Popen(cmd,
+                          stdout = subprocess.PIPE,
+                          stderr = subprocess.PIPE,
+                          env = env)
+  out = proc.stdout.read()
+  proc.stdout.close()
+
+  err = proc.stderr.read()
+  proc.stderr.close()
+
+  if proc.wait() != 0:
+    print >>sys.stderr
+    print >>sys.stderr, out
+    print >>sys.stderr, err
+    print >>sys.stderr
+    return False
+  return True