diff --git a/bin/commit-hook b/bin/commit-hook
new file mode 120000
index 0000000000..9eda12cfb7
--- /dev/null
+++ b/bin/commit-hook
@@ -0,0 +1 @@
+../scripts/repository/commit_hook.php
\ No newline at end of file
diff --git a/scripts/repository/commit_hook.php b/scripts/repository/commit_hook.php
new file mode 100755
index 0000000000..fbd93b1797
--- /dev/null
+++ b/scripts/repository/commit_hook.php
@@ -0,0 +1,56 @@
+#!/usr/bin/env php
+<?php
+
+$root = dirname(dirname(dirname(__FILE__)));
+require_once $root.'/scripts/__init_script__.php';
+
+$username = getenv('PHABRICATOR_USER');
+if (!$username) {
+  throw new Exception(pht('usage: define PHABRICATOR_USER in environment'));
+}
+
+$user = id(new PhabricatorPeopleQuery())
+  ->setViewer(PhabricatorUser::getOmnipotentUser())
+  ->withUsernames(array($username))
+  ->executeOne();
+if (!$user) {
+  throw new Exception(pht('No such user "%s"!', $username));
+}
+
+if ($argc < 2) {
+  throw new Exception(pht('usage: commit-hook <callsign>'));
+}
+
+$repository = id(new PhabricatorRepositoryQuery())
+  ->setViewer($user)
+  ->withCallsigns(array($argv[1]))
+  ->requireCapabilities(
+    array(
+      // This capability check is redundant, but can't hurt.
+      PhabricatorPolicyCapability::CAN_VIEW,
+      DiffusionCapabilityPush::CAPABILITY,
+    ))
+  ->executeOne();
+
+if (!$repository) {
+  throw new Exception(pht('No such repository "%s"!', $callsign));
+}
+
+if (!$repository->isHosted()) {
+  // This should be redundant too, but double check just in case.
+  throw new Exception(pht('Repository "%s" is not hosted!', $callsign));
+}
+
+$stdin = @file_get_contents('php://stdin');
+if ($stdin === false) {
+  throw new Exception(pht('Failed to read stdin!'));
+}
+
+$engine = id(new DiffusionCommitHookEngine())
+  ->setViewer($user)
+  ->setRepository($repository)
+  ->setStdin($stdin);
+
+$err = $engine->execute();
+
+exit($err);
diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index 5dddf8cc9a..88ab3d46de 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -474,6 +474,7 @@ phutil_register_library_map(array(
     'DiffusionCommitChangeTableView' => 'applications/diffusion/view/DiffusionCommitChangeTableView.php',
     'DiffusionCommitController' => 'applications/diffusion/controller/DiffusionCommitController.php',
     'DiffusionCommitEditController' => 'applications/diffusion/controller/DiffusionCommitEditController.php',
+    'DiffusionCommitHookEngine' => 'applications/diffusion/engine/DiffusionCommitHookEngine.php',
     'DiffusionCommitParentsQuery' => 'applications/diffusion/query/parents/DiffusionCommitParentsQuery.php',
     'DiffusionCommitQuery' => 'applications/diffusion/query/DiffusionCommitQuery.php',
     'DiffusionCommitTagsController' => 'applications/diffusion/controller/DiffusionCommitTagsController.php',
@@ -2797,6 +2798,7 @@ phutil_register_library_map(array(
     'DiffusionCommitChangeTableView' => 'DiffusionView',
     'DiffusionCommitController' => 'DiffusionController',
     'DiffusionCommitEditController' => 'DiffusionController',
+    'DiffusionCommitHookEngine' => 'Phobject',
     'DiffusionCommitParentsQuery' => 'DiffusionQuery',
     'DiffusionCommitQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
     'DiffusionCommitTagsController' => 'DiffusionController',
diff --git a/src/applications/diffusion/controller/DiffusionServeController.php b/src/applications/diffusion/controller/DiffusionServeController.php
index 7bb19e47ce..de7c2e1eb2 100644
--- a/src/applications/diffusion/controller/DiffusionServeController.php
+++ b/src/applications/diffusion/controller/DiffusionServeController.php
@@ -318,6 +318,7 @@ final class DiffusionServeController extends DiffusionController {
       'PATH_INFO' => $request_path,
 
       'REMOTE_USER' => $viewer->getUsername(),
+      'PHABRICATOR_USER' => $viewer->getUsername(),
 
       // TODO: Set these correctly.
       // GIT_COMMITTER_NAME
diff --git a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
new file mode 100644
index 0000000000..ca24d0271b
--- /dev/null
+++ b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
@@ -0,0 +1,76 @@
+<?php
+
+final class DiffusionCommitHookEngine extends Phobject {
+
+  private $viewer;
+  private $repository;
+  private $stdin;
+
+  public function setStdin($stdin) {
+    $this->stdin = $stdin;
+    return $this;
+  }
+
+  public function getStdin() {
+    return $this->stdin;
+  }
+
+  public function setRepository(PhabricatorRepository $repository) {
+    $this->repository = $repository;
+    return $this;
+  }
+
+  public function getRepository() {
+    return $this->repository;
+  }
+
+  public function setViewer(PhabricatorUser $viewer) {
+    $this->viewer = $viewer;
+    return $this;
+  }
+
+  public function getViewer() {
+    return $this->viewer;
+  }
+
+  public function execute() {
+    $type = $this->getRepository()->getVersionControlSystem();
+    switch ($type) {
+      case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
+        $err = $this->executeGitHook();
+        break;
+      default:
+        throw new Exception(pht('Unsupported repository type "%s"!', $type));
+    }
+
+    return $err;
+  }
+
+  private function executeGitHook() {
+    $updates = $this->parseGitUpdates($this->getStdin());
+
+    // TODO: Do useful things.
+
+    return 0;
+  }
+
+  private function parseGitUpdates($stdin) {
+    $updates = array();
+
+    $lines = phutil_split_lines($stdin, $retain_endings = false);
+    foreach ($lines as $line) {
+      $parts = explode(' ', $line, 3);
+      if (count($parts) != 3) {
+        throw new Exception(pht('Expected "old new ref", got "%s".', $line));
+      }
+      $updates[] = array(
+        'old' => $parts[0],
+        'new' => $parts[1],
+        'ref' => $parts[2],
+      );
+    }
+
+    return $updates;
+  }
+
+}
diff --git a/src/applications/diffusion/ssh/DiffusionSSHGitReceivePackWorkflow.php b/src/applications/diffusion/ssh/DiffusionSSHGitReceivePackWorkflow.php
index 4c379c4638..d2c59dee79 100644
--- a/src/applications/diffusion/ssh/DiffusionSSHGitReceivePackWorkflow.php
+++ b/src/applications/diffusion/ssh/DiffusionSSHGitReceivePackWorkflow.php
@@ -25,7 +25,8 @@ final class DiffusionSSHGitReceivePackWorkflow
     $command = csprintf('git-receive-pack %s', $repository->getLocalPath());
     $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
 
-    $future = new ExecFuture('%C', $command);
+    $future = id(new ExecFuture('%C', $command))
+      ->setEnv($this->getEnvironment());
 
     $err = $this->newPassthruCommand()
       ->setIOChannel($this->getIOChannel())
diff --git a/src/applications/diffusion/ssh/DiffusionSSHGitUploadPackWorkflow.php b/src/applications/diffusion/ssh/DiffusionSSHGitUploadPackWorkflow.php
index 0ccfb95508..242c9d0c7f 100644
--- a/src/applications/diffusion/ssh/DiffusionSSHGitUploadPackWorkflow.php
+++ b/src/applications/diffusion/ssh/DiffusionSSHGitUploadPackWorkflow.php
@@ -22,7 +22,8 @@ final class DiffusionSSHGitUploadPackWorkflow
     $command = csprintf('git-upload-pack -- %s', $repository->getLocalPath());
     $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
 
-    $future = new ExecFuture('%C', $command);
+    $future = id(new ExecFuture('%C', $command))
+      ->setEnv($this->getEnvironment());
 
     $err = $this->newPassthruCommand()
       ->setIOChannel($this->getIOChannel())
diff --git a/src/applications/diffusion/ssh/DiffusionSSHWorkflow.php b/src/applications/diffusion/ssh/DiffusionSSHWorkflow.php
index 954e01fd1a..be1a1d55fd 100644
--- a/src/applications/diffusion/ssh/DiffusionSSHWorkflow.php
+++ b/src/applications/diffusion/ssh/DiffusionSSHWorkflow.php
@@ -17,6 +17,12 @@ abstract class DiffusionSSHWorkflow extends PhabricatorSSHWorkflow {
     return $this->args;
   }
 
+  public function getEnvironment() {
+    return array(
+      'PHABRICATOR_USER' => $this->getUser()->getUsername(),
+    );
+  }
+
   abstract protected function executeRepositoryOperations();
 
   protected function writeError($message) {
diff --git a/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php b/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
index a7ef8b7cd1..1ab6818a46 100644
--- a/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
+++ b/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
@@ -83,11 +83,15 @@ final class PhabricatorRepositoryPullEngine
         }
       } else {
         if ($repository->isHosted()) {
-          $this->logPull(
-            pht(
-              "Repository '%s' is hosted, so Phabricator does not pull ".
-              "updates for it.",
-              $callsign));
+          if ($is_git) {
+            $this->installGitHook();
+          } else {
+            $this->logPull(
+              pht(
+                "Repository '%s' is hosted, so Phabricator does not pull ".
+                "updates for it.",
+                $callsign));
+          }
         } else {
           $this->logPull(
             pht(
@@ -146,6 +150,22 @@ final class PhabricatorRepositoryPullEngine
       ));
   }
 
+  private function installHook($path) {
+    $this->log('%s', pht('Installing commit hook to "%s"...', $path));
+
+    $repository = $this->getRepository();
+    $callsign = $repository->getCallsign();
+
+    $root = dirname(phutil_get_library_root('phabricator'));
+    $bin = $root.'/bin/commit-hook';
+    $cmd = csprintf('exec -- %s %s', $bin, $callsign);
+
+    $hook = "#!/bin/sh\n{$cmd}\n";
+
+    Filesystem::writeFile($path, $hook);
+    Filesystem::changePermissions($path, 0755);
+  }
+
 
 /* -(  Pulling Git Working Copies  )----------------------------------------- */
 
@@ -279,6 +299,23 @@ final class PhabricatorRepositoryPullEngine
   }
 
 
+  /**
+   * @task git
+   */
+  private function installGitHook() {
+    $repository = $this->getRepository();
+    $path = $repository->getLocalPath();
+
+    if ($repository->isWorkingCopyBare()) {
+      $path .= 'hooks/pre-receive';
+    } else {
+      $path .= '.git/hooks/pre-receive';
+    }
+
+    $this->installHook($path);
+  }
+
+
 /* -(  Pulling Mercurial Working Copies  )----------------------------------- */
 
 
@@ -357,4 +394,5 @@ final class PhabricatorRepositoryPullEngine
     execx('svnadmin create -- %s', $path);
   }
 
+
 }