diff --git a/src/applications/harbormaster/conduit/ConduitAPI_harbormaster_sendmessage_Method.php b/src/applications/harbormaster/conduit/ConduitAPI_harbormaster_sendmessage_Method.php
--- a/src/applications/harbormaster/conduit/ConduitAPI_harbormaster_sendmessage_Method.php
+++ b/src/applications/harbormaster/conduit/ConduitAPI_harbormaster_sendmessage_Method.php
@@ -43,6 +43,13 @@
       ->setType($message_type)
       ->save();
 
+    // If the build has completely paused because all steps are blocked on
+    // waiting targets, this will resume it.
+    id(new HarbormasterBuildEngine())
+      ->setViewer($viewer)
+      ->setBuild($build_target->getBuild())
+      ->continueBuild();
+
     return null;
   }
 
diff --git a/src/applications/harbormaster/controller/HarbormasterBuildableViewController.php b/src/applications/harbormaster/controller/HarbormasterBuildableViewController.php
--- a/src/applications/harbormaster/controller/HarbormasterBuildableViewController.php
+++ b/src/applications/harbormaster/controller/HarbormasterBuildableViewController.php
@@ -251,15 +251,27 @@
           switch ($target->getTargetStatus()) {
             case HarbormasterBuildTarget::STATUS_PENDING:
               $icon = 'time-green';
+              $status_name = pht('Pending');
+              break;
+            case HarbormasterBuildTarget::STATUS_BUILDING:
+              $icon = 'right-green';
+              $status_name = pht('Building');
+              break;
+            case HarbormasterBuildTarget::STATUS_WAITING:
+              $icon = 'time-orange';
+              $status_name = pht('Waiting');
               break;
             case HarbormasterBuildTarget::STATUS_PASSED:
               $icon = 'accept-green';
+              $status_name = pht('Passed');
               break;
             case HarbormasterBuildTarget::STATUS_FAILED:
               $icon = 'reject-red';
+              $status_name = pht('Failed');
               break;
             default:
               $icon = 'question';
+              $status_name = pht('Unknown');
               break;
           }
 
@@ -272,7 +284,7 @@
 
           $target_list->addItem(
             id(new PHUIStatusItemView())
-              ->setIcon($icon)
+              ->setIcon($icon, $status_name)
               ->setTarget(pht('Target %d', $target->getID()))
               ->setNote($name));
         }
diff --git a/src/applications/harbormaster/customfield/HarbormasterBuildStepCoreCustomField.php b/src/applications/harbormaster/customfield/HarbormasterBuildStepCoreCustomField.php
--- a/src/applications/harbormaster/customfield/HarbormasterBuildStepCoreCustomField.php
+++ b/src/applications/harbormaster/customfield/HarbormasterBuildStepCoreCustomField.php
@@ -9,7 +9,41 @@
   }
 
   public function createFields($object) {
-    $specs = $object->getStepImplementation()->getFieldSpecifications();
+    $impl = $object->getStepImplementation();
+    $specs = $impl->getFieldSpecifications();
+
+    if ($impl->supportsWaitForMessage()) {
+      $specs['builtin.next-steps-header'] = array(
+        'type' => 'header',
+        'name' => pht('Next Steps'),
+      );
+
+      $specs['builtin.wait-for-message'] = array(
+        'type' => 'select',
+        'name' => pht('When Complete'),
+        'instructions' => pht(
+          'After completing this build step Harbormaster can continue the '.
+          'build normally, or it can pause the build and wait for a message. '.
+          'If you are using this build step to trigger some work in an '.
+          'external system, you may want to have Phabricator wait for that '.
+          'system to perform the work and report results back.'.
+          "\n\n".
+          'If you select **Continue Build Normally**, the build plan will '.
+          'proceed once this step finishes.'.
+          "\n\n".
+          'If you select **Wait For Message**, the build plan will pause '.
+          'indefinitely once this step finishes. To resume the build, an '.
+          'external system must call `harbormaster.sendmessage` with the '.
+          'build target PHID, and either `"pass"` or `"fail"` to indicate '.
+          'the result for this step. After the result is recorded, the build '.
+          'plan will resume.'),
+        'options' => array(
+          '' => pht('Continue Build Normally'),
+          'wait' => pht('Wait For Message'),
+        ),
+      );
+    }
+
     return PhabricatorStandardCustomField::buildStandardFields($this, $specs);
   }
 
diff --git a/src/applications/harbormaster/engine/HarbormasterBuildEngine.php b/src/applications/harbormaster/engine/HarbormasterBuildEngine.php
--- a/src/applications/harbormaster/engine/HarbormasterBuildEngine.php
+++ b/src/applications/harbormaster/engine/HarbormasterBuildEngine.php
@@ -127,6 +127,9 @@
       ->setViewer($this->getViewer())
       ->withBuildPHIDs(array($build->getPHID()))
       ->execute();
+
+    $this->updateWaitingTargets($targets);
+
     $targets = mgroup($targets, 'getBuildStepPHID');
 
     $steps = id(new HarbormasterBuildStepQuery())
@@ -134,15 +137,35 @@
       ->withBuildPlanPHIDs(array($build->getBuildPlan()->getPHID()))
       ->execute();
 
-    // Identify steps which are complete.
+    // Identify steps which are in various states.
 
+    $queued = array();
+    $underway = array();
+    $waiting = array();
     $complete = array();
     $failed = array();
-    $waiting = array();
     foreach ($steps as $step) {
       $step_targets = idx($targets, $step->getPHID(), array());
 
       if ($step_targets) {
+        $is_queued = false;
+
+        $is_underway = false;
+        foreach ($step_targets as $target) {
+          if ($target->isUnderway()) {
+            $is_underway = true;
+            break;
+          }
+        }
+
+        $is_waiting = false;
+        foreach ($step_targets as $target) {
+          if ($target->isWaiting()) {
+            $is_waiting = true;
+            break;
+          }
+        }
+
         $is_complete = true;
         foreach ($step_targets as $target) {
           if (!$target->isComplete()) {
@@ -158,25 +181,33 @@
             break;
           }
         }
-
-        $is_waiting = false;
       } else {
+        $is_queued = true;
+        $is_underway = false;
+        $is_waiting = false;
         $is_complete = false;
         $is_failed = false;
-        $is_waiting = true;
       }
 
-      if ($is_complete) {
-        $complete[$step->getPHID()] = true;
+      if ($is_queued) {
+        $queued[$step->getPHID()] = true;
       }
 
-      if ($is_failed) {
-        $failed[$step->getPHID()] = true;
+      if ($is_underway) {
+        $underway[$step->getPHID()] = true;
       }
 
       if ($is_waiting) {
         $waiting[$step->getPHID()] = true;
       }
+
+      if ($is_complete) {
+        $complete[$step->getPHID()] = true;
+      }
+
+      if ($is_failed) {
+        $failed[$step->getPHID()] = true;
+      }
     }
 
     // If any step failed, fail the whole build, then bail.
@@ -209,7 +240,7 @@
         $dependencies = array();
       }
 
-      if (isset($waiting[$step->getPHID()])) {
+      if (isset($queued[$step->getPHID()])) {
         $can_run = true;
         foreach ($dependencies as $dependency) {
           if (empty($complete[$dependency->getPHID()])) {
@@ -226,9 +257,9 @@
       $previous_step = $step;
     }
 
-    if (!$runnable) {
+    if (!$runnable && !$waiting && !$underway) {
       // TODO: This means the build is deadlocked, probably? It should not
-      // normally be possible, but we should communicate it more clearly.
+      // normally be possible yet, but we should communicate it more clearly.
       $build->setBuildStatus(HarbormasterBuild::STATUS_FAILED);
       $build->save();
       return;
@@ -243,6 +274,60 @@
 
       $this->queueNewBuildTarget($target);
     }
+
+  }
+
+
+  /**
+   * Process messages which were sent to these targets, kicking applicable
+   * targets out of "Waiting" and into either "Passed" or "Failed".
+   *
+   * @param list<HarbormasterBuildTarget> List of targets to process.
+   * @return void
+   */
+  private function updateWaitingTargets(array $targets) {
+    assert_instances_of($targets, 'HarbormasterBuildTarget');
+
+    // We only care about messages for targets which are actually in a waiting
+    // state.
+    $waiting_targets = array();
+    foreach ($targets as $target) {
+      if ($target->isWaiting()) {
+        $waiting_targets[$target->getPHID()] = $target;
+      }
+    }
+
+    if (!$waiting_targets) {
+      return;
+    }
+
+    $messages = id(new HarbormasterBuildMessageQuery())
+      ->setViewer($this->getViewer())
+      ->withBuildTargetPHIDs(array_keys($waiting_targets))
+      ->withConsumed(false)
+      ->execute();
+
+    foreach ($messages as $message) {
+      $target = $waiting_targets[$message->getBuildTargetPHID()];
+
+      $new_status = null;
+      switch ($message->getType()) {
+        case 'pass':
+          $new_status = HarbormasterBuildTarget::STATUS_PASSED;
+          break;
+        case 'fail':
+          $new_status = HarbormasterBuildTarget::STATUS_FAILED;
+          break;
+      }
+
+      if ($new_status !== null) {
+        $message->setIsConsumed(true);
+        $message->save();
+
+        $target->setTargetStatus($new_status);
+        $target->save();
+      }
+    }
   }
 
 }
diff --git a/src/applications/harbormaster/query/HarbormasterBuildMessageQuery.php b/src/applications/harbormaster/query/HarbormasterBuildMessageQuery.php
--- a/src/applications/harbormaster/query/HarbormasterBuildMessageQuery.php
+++ b/src/applications/harbormaster/query/HarbormasterBuildMessageQuery.php
@@ -83,7 +83,7 @@
       $where[] = qsprintf(
         $conn_r,
         'isConsumed = %d',
-        (int)$this->isConsumed);
+        (int)$this->consumed);
     }
 
     $where[] = $this->buildPagingClause($conn_r);
diff --git a/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
@@ -196,4 +196,16 @@
     }
   }
 
+  public function supportsWaitForMessage() {
+    return false;
+  }
+
+  public function shouldWaitForMessage(HarbormasterBuildTarget $target) {
+    if (!$this->supportsWaitForMessage()) {
+      return false;
+    }
+
+    return (bool)$target->getDetail('builtin.wait-for-message');
+  }
+
 }
diff --git a/src/applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterHTTPRequestBuildStepImplementation.php
@@ -99,4 +99,8 @@
     );
   }
 
+  public function supportsWaitForMessage() {
+    return true;
+  }
+
 }
diff --git a/src/applications/harbormaster/storage/build/HarbormasterBuildTarget.php b/src/applications/harbormaster/storage/build/HarbormasterBuildTarget.php
--- a/src/applications/harbormaster/storage/build/HarbormasterBuildTarget.php
+++ b/src/applications/harbormaster/storage/build/HarbormasterBuildTarget.php
@@ -11,6 +11,8 @@
   protected $targetStatus;
 
   const STATUS_PENDING = 'target/pending';
+  const STATUS_BUILDING = 'target/building';
+  const STATUS_WAITING = 'target/waiting';
   const STATUS_PASSED = 'target/passed';
   const STATUS_FAILED = 'target/failed';
 
@@ -128,6 +130,26 @@
   }
 
 
+  public function isWaiting() {
+    switch ($this->getTargetStatus()) {
+      case self::STATUS_WAITING:
+        return true;
+    }
+
+    return false;
+  }
+
+  public function isUnderway() {
+    switch ($this->getTargetStatus()) {
+      case self::STATUS_PENDING:
+      case self::STATUS_BUILDING:
+        return true;
+    }
+
+    return false;
+  }
+
+
 /* -(  PhabricatorPolicyInterface  )----------------------------------------- */
 
 
diff --git a/src/applications/harbormaster/worker/HarbormasterTargetWorker.php b/src/applications/harbormaster/worker/HarbormasterTargetWorker.php
--- a/src/applications/harbormaster/worker/HarbormasterTargetWorker.php
+++ b/src/applications/harbormaster/worker/HarbormasterTargetWorker.php
@@ -36,9 +36,21 @@
     $viewer = $this->getViewer();
 
     try {
+      $status_pending = HarbormasterBuildTarget::STATUS_PENDING;
+      if ($target->getTargetStatus() == $status_pending) {
+        $target->setTargetStatus(HarbormasterBuildTarget::STATUS_BUILDING);
+        $target->save();
+      }
+
       $implementation = $target->getImplementation();
       $implementation->execute($build, $target);
-      $target->setTargetStatus(HarbormasterBuildTarget::STATUS_PASSED);
+
+      $next_status = HarbormasterBuildTarget::STATUS_PASSED;
+      if ($implementation->shouldWaitForMessage($target)) {
+        $next_status = HarbormasterBuildTarget::STATUS_WAITING;
+      }
+
+      $target->setTargetStatus($next_status);
       $target->save();
     } catch (Exception $ex) {
       phlog($ex);