diff --git a/docs/manifest-format.txt b/docs/manifest-format.txt
index 2e1c8c3..c76df80 100644
--- a/docs/manifest-format.txt
+++ b/docs/manifest-format.txt
@@ -25,7 +25,8 @@
                         default?,
                         manifest-server?,
                         remove-project*,
-                        project*)>
+                        project*,
+                        repo-hooks?)>
   
     <!ELEMENT notice (#PCDATA)>
   
@@ -49,6 +50,10 @@
   
     <!ELEMENT remove-project (EMPTY)>
     <!ATTLIST remove-project name  CDATA #REQUIRED>
+
+    <!ELEMENT repo-hooks (EMPTY)>
+    <!ATTLIST repo-hooks in-project CDATA #REQUIRED>
+    <!ATTLIST repo-hooks enabled-list CDATA #REQUIRED>
   ]>
 
 A description of the elements and their attributes follows.
diff --git a/error.py b/error.py
index cb3b725..5238158 100644
--- a/error.py
+++ b/error.py
@@ -75,3 +75,10 @@
   """
   def __init__(self, extra_args=[]):
     self.extra_args = extra_args
+
+class HookError(Exception):
+  """Thrown if a 'repo-hook' could not be run.
+
+  The common case is that the file wasn't present when we tried to run it.
+  """
+  pass
diff --git a/manifest_xml.py b/manifest_xml.py
index 0103cf5..0e6421f 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -171,6 +171,14 @@
         ce.setAttribute('dest', c.dest)
         e.appendChild(ce)
 
+    if self._repo_hooks_project:
+      root.appendChild(doc.createTextNode(''))
+      e = doc.createElement('repo-hooks')
+      e.setAttribute('in-project', self._repo_hooks_project.name)
+      e.setAttribute('enabled-list',
+                     ' '.join(self._repo_hooks_project.enabled_repo_hooks))
+      root.appendChild(e)
+
     doc.writexml(fd, '', '  ', '\n', 'UTF-8')
 
   @property
@@ -189,6 +197,11 @@
     return self._default
 
   @property
+  def repo_hooks_project(self):
+    self._Load()
+    return self._repo_hooks_project
+
+  @property
   def notice(self):
     self._Load()
     return self._notice
@@ -207,6 +220,7 @@
     self._projects = {}
     self._remotes = {}
     self._default = None
+    self._repo_hooks_project = None
     self._notice = None
     self.branch = None
     self._manifest_server = None
@@ -239,15 +253,15 @@
   def _ParseManifest(self, is_root_file):
     root = xml.dom.minidom.parse(self.manifestFile)
     if not root or not root.childNodes:
-      raise ManifestParseError, \
-            "no root node in %s" % \
-            self.manifestFile
+      raise ManifestParseError(
+          "no root node in %s" %
+          self.manifestFile)
 
     config = root.childNodes[0]
     if config.nodeName != 'manifest':
-      raise ManifestParseError, \
-            "no <manifest> in %s" % \
-            self.manifestFile
+      raise ManifestParseError(
+          "no <manifest> in %s" %
+          self.manifestFile)
 
     for node in config.childNodes:
       if node.nodeName == 'remove-project':
@@ -255,25 +269,30 @@
         try:
           del self._projects[name]
         except KeyError:
-          raise ManifestParseError, \
-                'project %s not found' % \
-                (name)
+          raise ManifestParseError(
+              'project %s not found' %
+              (name))
+
+        # If the manifest removes the hooks project, treat it as if it deleted
+        # the repo-hooks element too.
+        if self._repo_hooks_project and (self._repo_hooks_project.name == name):
+          self._repo_hooks_project = None
 
     for node in config.childNodes:
       if node.nodeName == 'remote':
         remote = self._ParseRemote(node)
         if self._remotes.get(remote.name):
-          raise ManifestParseError, \
-                'duplicate remote %s in %s' % \
-                (remote.name, self.manifestFile)
+          raise ManifestParseError(
+              'duplicate remote %s in %s' %
+              (remote.name, self.manifestFile))
         self._remotes[remote.name] = remote
 
     for node in config.childNodes:
       if node.nodeName == 'default':
         if self._default is not None:
-          raise ManifestParseError, \
-                'duplicate default in %s' % \
-                (self.manifestFile)
+          raise ManifestParseError(
+              'duplicate default in %s' %
+              (self.manifestFile))
         self._default = self._ParseDefault(node)
     if self._default is None:
       self._default = _Default()
@@ -281,29 +300,52 @@
     for node in config.childNodes:
       if node.nodeName == 'notice':
         if self._notice is not None:
-          raise ManifestParseError, \
-                'duplicate notice in %s' % \
-                (self.manifestFile)
+          raise ManifestParseError(
+              'duplicate notice in %s' %
+              (self.manifestFile))
         self._notice = self._ParseNotice(node)
 
     for node in config.childNodes:
       if node.nodeName == 'manifest-server':
         url = self._reqatt(node, 'url')
         if self._manifest_server is not None:
-            raise ManifestParseError, \
-                'duplicate manifest-server in %s' % \
-                (self.manifestFile)
+            raise ManifestParseError(
+                'duplicate manifest-server in %s' %
+                (self.manifestFile))
         self._manifest_server = url
 
     for node in config.childNodes:
       if node.nodeName == 'project':
         project = self._ParseProject(node)
         if self._projects.get(project.name):
-          raise ManifestParseError, \
-                'duplicate project %s in %s' % \
-                (project.name, self.manifestFile)
+          raise ManifestParseError(
+              'duplicate project %s in %s' %
+              (project.name, self.manifestFile))
         self._projects[project.name] = project
 
+    for node in config.childNodes:
+      if node.nodeName == 'repo-hooks':
+        # Get the name of the project and the (space-separated) list of enabled.
+        repo_hooks_project = self._reqatt(node, 'in-project')
+        enabled_repo_hooks = self._reqatt(node, 'enabled-list').split()
+
+        # Only one project can be the hooks project
+        if self._repo_hooks_project is not None:
+          raise ManifestParseError(
+              'duplicate repo-hooks in %s' %
+              (self.manifestFile))
+
+        # Store a reference to the Project.
+        try:
+          self._repo_hooks_project = self._projects[repo_hooks_project]
+        except KeyError:
+          raise ManifestParseError(
+              'project %s not found for repo-hooks' %
+              (repo_hooks_project))
+
+        # Store the enabled hooks in the Project object.
+        self._repo_hooks_project.enabled_repo_hooks = enabled_repo_hooks
+
   def _AddMetaProjectMirror(self, m):
     name = None
     m_url = m.GetRemote(m.remote.name).url
diff --git a/project.py b/project.py
index 37f6d36..49633f7 100644
--- a/project.py
+++ b/project.py
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import traceback
 import errno
 import filecmp
 import os
@@ -24,7 +25,7 @@
 from color import Coloring
 from git_command import GitCommand
 from git_config import GitConfig, IsId
-from error import GitError, ImportError, UploadError
+from error import GitError, HookError, ImportError, UploadError
 from error import ManifestInvalidRevisionError
 
 from git_refs import GitRefs, HEAD, R_HEADS, R_TAGS, R_PUB, R_M
@@ -234,6 +235,249 @@
     self.url = url
     self.review = review
 
+class RepoHook(object):
+  """A RepoHook contains information about a script to run as a hook.
+
+  Hooks are used to run a python script before running an upload (for instance,
+  to run presubmit checks).  Eventually, we may have hooks for other actions.
+
+  This shouldn't be confused with files in the 'repo/hooks' directory.  Those
+  files are copied into each '.git/hooks' folder for each project.  Repo-level
+  hooks are associated instead with repo actions.
+
+  Hooks are always python.  When a hook is run, we will load the hook into the
+  interpreter and execute its main() function.
+  """
+  def __init__(self,
+               hook_type,
+               hooks_project,
+               topdir,
+               abort_if_user_denies=False):
+    """RepoHook constructor.
+
+    Params:
+      hook_type: A string representing the type of hook.  This is also used
+          to figure out the name of the file containing the hook.  For
+          example: 'pre-upload'.
+      hooks_project: The project containing the repo hooks.  If you have a
+          manifest, this is manifest.repo_hooks_project.  OK if this is None,
+          which will make the hook a no-op.
+      topdir: Repo's top directory (the one containing the .repo directory).
+          Scripts will run with CWD as this directory.  If you have a manifest,
+          this is manifest.topdir
+      abort_if_user_denies: If True, we'll throw a HookError() if the user
+          doesn't allow us to run the hook.
+    """
+    self._hook_type = hook_type
+    self._hooks_project = hooks_project
+    self._topdir = topdir
+    self._abort_if_user_denies = abort_if_user_denies
+
+    # Store the full path to the script for convenience.
+    if self._hooks_project:
+      self._script_fullpath = os.path.join(self._hooks_project.worktree,
+                                           self._hook_type + '.py')
+    else:
+      self._script_fullpath = None
+
+  def _GetHash(self):
+    """Return a hash of the contents of the hooks directory.
+
+    We'll just use git to do this.  This hash has the property that if anything
+    changes in the directory we will return a different has.
+
+    SECURITY CONSIDERATION:
+      This hash only represents the contents of files in the hook directory, not
+      any other files imported or called by hooks.  Changes to imported files
+      can change the script behavior without affecting the hash.
+
+    Returns:
+      A string representing the hash.  This will always be ASCII so that it can
+      be printed to the user easily.
+    """
+    assert self._hooks_project, "Must have hooks to calculate their hash."
+
+    # We will use the work_git object rather than just calling GetRevisionId().
+    # That gives us a hash of the latest checked in version of the files that
+    # the user will actually be executing.  Specifically, GetRevisionId()
+    # doesn't appear to change even if a user checks out a different version
+    # of the hooks repo (via git checkout) nor if a user commits their own revs.
+    #
+    # NOTE: Local (non-committed) changes will not be factored into this hash.
+    # I think this is OK, since we're really only worried about warning the user
+    # about upstream changes.
+    return self._hooks_project.work_git.rev_parse('HEAD')
+
+  def _GetMustVerb(self):
+    """Return 'must' if the hook is required; 'should' if not."""
+    if self._abort_if_user_denies:
+      return 'must'
+    else:
+      return 'should'
+
+  def _CheckForHookApproval(self):
+    """Check to see whether this hook has been approved.
+
+    We'll look at the hash of all of the hooks.  If this matches the hash that
+    the user last approved, we're done.  If it doesn't, we'll ask the user
+    about approval.
+
+    Note that we ask permission for each individual hook even though we use
+    the hash of all hooks when detecting changes.  We'd like the user to be
+    able to approve / deny each hook individually.  We only use the hash of all
+    hooks because there is no other easy way to detect changes to local imports.
+
+    Returns:
+      True if this hook is approved to run; False otherwise.
+
+    Raises:
+      HookError: Raised if the user doesn't approve and abort_if_user_denies
+          was passed to the consturctor.
+    """
+    hooks_dir = self._hooks_project.worktree
+    hooks_config = self._hooks_project.config
+    git_approval_key = 'repo.hooks.%s.approvedhash' % self._hook_type
+
+    # Get the last hash that the user approved for this hook; may be None.
+    old_hash = hooks_config.GetString(git_approval_key)
+
+    # Get the current hash so we can tell if scripts changed since approval.
+    new_hash = self._GetHash()
+
+    if old_hash is not None:
+      # User previously approved hook and asked not to be prompted again.
+      if new_hash == old_hash:
+        # Approval matched.  We're done.
+        return True
+      else:
+        # Give the user a reason why we're prompting, since they last told
+        # us to "never ask again".
+        prompt = 'WARNING: Scripts have changed since %s was allowed.\n\n' % (
+            self._hook_type)
+    else:
+      prompt = ''
+
+    # Prompt the user if we're not on a tty; on a tty we'll assume "no".
+    if sys.stdout.isatty():
+      prompt += ('Repo %s run the script:\n'
+                 '  %s\n'
+                 '\n'
+                 'Do you want to allow this script to run '
+                 '(yes/yes-never-ask-again/NO)? ') % (
+                 self._GetMustVerb(), self._script_fullpath)
+      response = raw_input(prompt).lower()
+      print
+
+      # User is doing a one-time approval.
+      if response in ('y', 'yes'):
+        return True
+      elif response == 'yes-never-ask-again':
+        hooks_config.SetString(git_approval_key, new_hash)
+        return True
+
+    # For anything else, we'll assume no approval.
+    if self._abort_if_user_denies:
+      raise HookError('You must allow the %s hook or use --no-verify.' %
+                      self._hook_type)
+
+    return False
+
+  def _ExecuteHook(self, **kwargs):
+    """Actually execute the given hook.
+
+    This will run the hook's 'main' function in our python interpreter.
+
+    Args:
+      kwargs: Keyword arguments to pass to the hook.  These are often specific
+          to the hook type.  For instance, pre-upload hooks will contain
+          a project_list.
+    """
+    # Keep sys.path and CWD stashed away so that we can always restore them
+    # upon function exit.
+    orig_path = os.getcwd()
+    orig_syspath = sys.path
+
+    try:
+      # Always run hooks with CWD as topdir.
+      os.chdir(self._topdir)
+
+      # Put the hook dir as the first item of sys.path so hooks can do
+      # relative imports.  We want to replace the repo dir as [0] so
+      # hooks can't import repo files.
+      sys.path = [os.path.dirname(self._script_fullpath)] + sys.path[1:]
+
+      # Exec, storing global context in the context dict.  We catch exceptions
+      # and  convert to a HookError w/ just the failing traceback.
+      context = {}
+      try:
+        execfile(self._script_fullpath, context)
+      except Exception:
+        raise HookError('%s\nFailed to import %s hook; see traceback above.' % (
+                        traceback.format_exc(), self._hook_type))
+
+      # Running the script should have defined a main() function.
+      if 'main' not in context:
+        raise HookError('Missing main() in: "%s"' % self._script_fullpath)
+
+
+      # Add 'hook_should_take_kwargs' to the arguments to be passed to main.
+      # We don't actually want hooks to define their main with this argument--
+      # it's there to remind them that their hook should always take **kwargs.
+      # For instance, a pre-upload hook should be defined like:
+      #   def main(project_list, **kwargs):
+      #
+      # This allows us to later expand the API without breaking old hooks.
+      kwargs = kwargs.copy()
+      kwargs['hook_should_take_kwargs'] = True
+
+      # Call the main function in the hook.  If the hook should cause the
+      # build to fail, it will raise an Exception.  We'll catch that convert
+      # to a HookError w/ just the failing traceback.
+      try:
+        context['main'](**kwargs)
+      except Exception:
+        raise HookError('%s\nFailed to run main() for %s hook; see traceback '
+                        'above.' % (
+                        traceback.format_exc(), self._hook_type))
+    finally:
+      # Restore sys.path and CWD.
+      sys.path = orig_syspath
+      os.chdir(orig_path)
+
+  def Run(self, user_allows_all_hooks, **kwargs):
+    """Run the hook.
+
+    If the hook doesn't exist (because there is no hooks project or because
+    this particular hook is not enabled), this is a no-op.
+
+    Args:
+      user_allows_all_hooks: If True, we will never prompt about running the
+          hook--we'll just assume it's OK to run it.
+      kwargs: Keyword arguments to pass to the hook.  These are often specific
+          to the hook type.  For instance, pre-upload hooks will contain
+          a project_list.
+
+    Raises:
+      HookError: If there was a problem finding the hook or the user declined
+          to run a required hook (from _CheckForHookApproval).
+    """
+    # No-op if there is no hooks project or if hook is disabled.
+    if ((not self._hooks_project) or
+        (self._hook_type not in self._hooks_project.enabled_repo_hooks)):
+      return
+
+    # Bail with a nice error if we can't find the hook.
+    if not os.path.isfile(self._script_fullpath):
+      raise HookError('Couldn\'t find repo hook: "%s"' % self._script_fullpath)
+
+    # Make sure the user is OK with running the hook.
+    if (not user_allows_all_hooks) and (not self._CheckForHookApproval()):
+      return
+
+    # Run the hook with the same version of python we're using.
+    self._ExecuteHook(**kwargs)
+
+
 class Project(object):
   def __init__(self,
                manifest,
@@ -275,6 +519,10 @@
     self.bare_git = self._GitGetByExec(self, bare=True)
     self.bare_ref = GitRefs(gitdir)
 
+    # This will be filled in if a project is later identified to be the
+    # project containing repo hooks.
+    self.enabled_repo_hooks = []
+
   @property
   def Exists(self):
     return os.path.isdir(self.gitdir)
@@ -1457,6 +1705,22 @@
       return r
 
     def __getattr__(self, name):
+      """Allow arbitrary git commands using pythonic syntax.
+
+      This allows you to do things like:
+        git_obj.rev_parse('HEAD')
+
+      Since we don't have a 'rev_parse' method defined, the __getattr__ will
+      run.  We'll replace the '_' with a '-' and try to run a git command.
+      Any other arguments will be passed to the git command.
+
+      Args:
+        name: The name of the git command to call.  Any '_' characters will
+            be replaced with '-'.
+
+      Returns:
+        A callable object that will try to call git with the named command.
+      """
       name = name.replace('_', '-')
       def runner(*args):
         cmdv = [name]
diff --git a/subcmds/upload.py b/subcmds/upload.py
index 2082209..c561b8a 100644
--- a/subcmds/upload.py
+++ b/subcmds/upload.py
@@ -19,7 +19,8 @@
 
 from command import InteractiveCommand
 from editor import Editor
-from error import UploadError
+from error import HookError, UploadError
+from project import RepoHook
 
 UNUSUAL_COMMIT_THRESHOLD = 5
 
@@ -120,6 +121,29 @@
                  type='string',  action='append', dest='cc',
                  help='Also send email to these email addresses.')
 
+    # Options relating to upload hook.  Note that verify and no-verify are NOT
+    # opposites of each other, which is why they store to different locations.
+    # We are using them to match 'git commit' syntax.
+    #
+    # Combinations:
+    # - no-verify=False, verify=False (DEFAULT):
+    #   If stdout is a tty, can prompt about running upload hooks if needed.
+    #   If user denies running hooks, the upload is cancelled.  If stdout is
+    #   not a tty and we would need to prompt about upload hooks, upload is
+    #   cancelled.
+    # - no-verify=False, verify=True:
+    #   Always run upload hooks with no prompt.
+    # - no-verify=True, verify=False:
+    #   Never run upload hooks, but upload anyway (AKA bypass hooks).
+    # - no-verify=True, verify=True:
+    #   Invalid
+    p.add_option('--no-verify',
+                 dest='bypass_hooks', action='store_true',
+                 help='Do not run the upload hook.')
+    p.add_option('--verify',
+                 dest='allow_all_hooks', action='store_true',
+                 help='Run the upload hook without prompting.')
+
   def _SingleBranch(self, opt, branch, people):
     project = branch.project
     name = branch.name
@@ -313,17 +337,27 @@
     reviewers = []
     cc = []
 
+    for project in project_list:
+      avail = project.GetUploadableBranches()
+      if avail:
+        pending.append((project, avail))
+
+    if pending and (not opt.bypass_hooks):
+      hook = RepoHook('pre-upload', self.manifest.repo_hooks_project,
+                      self.manifest.topdir, abort_if_user_denies=True)
+      pending_proj_names = [project.name for (project, avail) in pending]
+      try:
+        hook.Run(opt.allow_all_hooks, project_list=pending_proj_names)
+      except HookError, e:
+        print >>sys.stderr, "ERROR: %s" % str(e)
+        return
+
     if opt.reviewers:
       reviewers = _SplitEmails(opt.reviewers)
     if opt.cc:
       cc = _SplitEmails(opt.cc)
     people = (reviewers,cc)
 
-    for project in project_list:
-      avail = project.GetUploadableBranches()
-      if avail:
-        pending.append((project, avail))
-
     if not pending:
       print >>sys.stdout, "no branches ready for upload"
     elif len(pending) == 1 and len(pending[0][1]) == 1: