diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -1108,6 +1108,7 @@
     'HarbormasterBuiltinBuildStepGroup' => 'applications/harbormaster/stepgroup/HarbormasterBuiltinBuildStepGroup.php',
     'HarbormasterCircleCIBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php',
     'HarbormasterCircleCIBuildableInterface' => 'applications/harbormaster/interface/HarbormasterCircleCIBuildableInterface.php',
+    'HarbormasterCircleCIHookController' => 'applications/harbormaster/controller/HarbormasterCircleCIHookController.php',
     'HarbormasterConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterConduitAPIMethod.php',
     'HarbormasterController' => 'applications/harbormaster/controller/HarbormasterController.php',
     'HarbormasterCreateArtifactConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterCreateArtifactConduitAPIMethod.php',
@@ -5338,6 +5339,7 @@
     'HarbormasterBuildableViewController' => 'HarbormasterController',
     'HarbormasterBuiltinBuildStepGroup' => 'HarbormasterBuildStepGroup',
     'HarbormasterCircleCIBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
+    'HarbormasterCircleCIHookController' => 'HarbormasterController',
     'HarbormasterConduitAPIMethod' => 'ConduitAPIMethod',
     'HarbormasterController' => 'PhabricatorController',
     'HarbormasterCreateArtifactConduitAPIMethod' => 'HarbormasterConduitAPIMethod',
diff --git a/src/applications/harbormaster/application/PhabricatorHarbormasterApplication.php b/src/applications/harbormaster/application/PhabricatorHarbormasterApplication.php
--- a/src/applications/harbormaster/application/PhabricatorHarbormasterApplication.php
+++ b/src/applications/harbormaster/application/PhabricatorHarbormasterApplication.php
@@ -91,6 +91,9 @@
         'lint/' => array(
           '(?P<id>\d+)/' => 'HarbormasterLintMessagesController',
         ),
+        'hook/' => array(
+          'circleci/' => 'HarbormasterCircleCIHookController',
+        ),
       ),
     );
   }
diff --git a/src/applications/harbormaster/controller/HarbormasterCircleCIHookController.php b/src/applications/harbormaster/controller/HarbormasterCircleCIHookController.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/controller/HarbormasterCircleCIHookController.php
@@ -0,0 +1,87 @@
+<?php
+
+final class HarbormasterCircleCIHookController
+  extends HarbormasterController {
+
+  public function shouldRequireLogin() {
+    return false;
+  }
+
+  /**
+   * @phutil-external-symbol class PhabricatorStartup
+   */
+  public function handleRequest(AphrontRequest $request) {
+    $raw_body = PhabricatorStartup::getRawInput();
+    $body = phutil_json_decode($raw_body);
+
+    $payload = $body['payload'];
+
+    $parameters = idx($payload, 'build_parameters');
+    if (!$parameters) {
+      $parameters = array();
+    }
+
+    $target_phid = idx($parameters, 'HARBORMASTER_BUILD_TARGET_PHID');
+
+    // NOTE: We'll get callbacks here for builds we triggered, but also for
+    // arbitrary builds the system executes for other reasons. So it's normal
+    // to get some notifications with no Build Target PHID. We just ignore
+    // these under the assumption that they're routine builds caused by events
+    // like branch updates.
+
+    if ($target_phid) {
+      $viewer = PhabricatorUser::getOmnipotentUser();
+      $target = id(new HarbormasterBuildTargetQuery())
+        ->setViewer($viewer)
+        ->withPHIDs(array($target_phid))
+        ->needBuildSteps(true)
+        ->executeOne();
+      if ($target) {
+        $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
+        $this->updateTarget($target, $payload);
+      }
+    }
+
+    $response = new AphrontWebpageResponse();
+    $response->setContent(pht("Request OK\n"));
+    return $response;
+  }
+
+  private function updateTarget(
+    HarbormasterBuildTarget $target,
+    array $payload) {
+
+    $step = $target->getBuildStep();
+    $impl = $step->getStepImplementation();
+    if (!($impl instanceof HarbormasterCircleCIBuildStepImplementation)) {
+      throw new Exception(
+        pht(
+          'Build target ("%s") has the wrong type of build step. Only '.
+          'CircleCI build steps may be updated via the CircleCI webhook.',
+          $target->getPHID()));
+    }
+
+    switch (idx($payload, 'status')) {
+      case 'success':
+      case 'fixed':
+        $message_type = HarbormasterMessageType::MESSAGE_PASS;
+        break;
+      default:
+        $message_type = HarbormasterMessageType::MESSAGE_FAIL;
+        break;
+    }
+
+    $viewer = PhabricatorUser::getOmnipotentUser();
+
+    $api_method = 'harbormaster.sendmessage';
+    $api_params = array(
+      'buildTargetPHID' => $target->getPHID(),
+      'type' => $message_type,
+    );
+
+    id(new ConduitCall($api_method, $api_params))
+      ->setUser($viewer)
+      ->execute();
+  }
+
+}
diff --git a/src/applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterCircleCIBuildStepImplementation.php
@@ -20,6 +20,9 @@
   }
 
   public function getEditInstructions() {
+    $hook_uri = '/harbormaster/hook/circleci/';
+    $hook_uri = PhabricatorEnv::getProductionURI($hook_uri);
+
     return pht(<<<EOTEXT
 WARNING: This build step is new and experimental!
 
@@ -38,7 +41,15 @@
 Webhook Configuration
 =====================
 
-IMPORTANT: This has not been implemented yet.
+Add this webhook to your `circle.yml` file to make CircleCI report results
+to Harbormaster. Until you install this hook, builds will hang waiting for
+a response from CircleCI.
+
+```lang=yml
+notify:
+  webhooks:
+    - url: %s
+```
 
 Environment
 ===========
@@ -50,7 +61,8 @@
 | `HARBORMASTER_BUILD_TARGET_PHID` | PHID of the Build Target.
 
 EOTEXT
-    );
+    ,
+    $hook_uri);
   }
 
   public static function getGitHubPath($uri) {
diff --git a/src/applications/harbormaster/storage/HarbormasterBuildMessage.php b/src/applications/harbormaster/storage/HarbormasterBuildMessage.php
--- a/src/applications/harbormaster/storage/HarbormasterBuildMessage.php
+++ b/src/applications/harbormaster/storage/HarbormasterBuildMessage.php
@@ -17,8 +17,13 @@
   private $buildTarget = self::ATTACHABLE;
 
   public static function initializeNewMessage(PhabricatorUser $actor) {
+    $actor_phid = $actor->getPHID();
+    if (!$actor_phid) {
+      $actor_phid = id(new PhabricatorHarbormasterApplication())->getPHID();
+    }
+
     return id(new HarbormasterBuildMessage())
-      ->setAuthorPHID($actor->getPHID())
+      ->setAuthorPHID($actor_phid)
       ->setIsConsumed(0);
   }