Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -2366,6 +2366,7 @@
     'SlowvoteRemarkupRule' => 'applications/slowvote/remarkup/SlowvoteRemarkupRule.php',
     'UploadArtifactBuildStepImplementation' => 'applications/harbormaster/step/UploadArtifactBuildStepImplementation.php',
     'VariableBuildStepImplementation' => 'applications/harbormaster/step/VariableBuildStepImplementation.php',
+    'WaitForPreviousBuildStepImplementation' => 'applications/harbormaster/step/WaitForPreviousBuildStepImplementation.php',
   ),
   'function' =>
   array(
@@ -5056,5 +5057,6 @@
     'SlowvoteRemarkupRule' => 'PhabricatorRemarkupRuleObject',
     'UploadArtifactBuildStepImplementation' => 'VariableBuildStepImplementation',
     'VariableBuildStepImplementation' => 'BuildStepImplementation',
+    'WaitForPreviousBuildStepImplementation' => 'BuildStepImplementation',
   ),
 ));
Index: src/applications/harbormaster/controller/HarbormasterBuildViewController.php
===================================================================
--- src/applications/harbormaster/controller/HarbormasterBuildViewController.php
+++ src/applications/harbormaster/controller/HarbormasterBuildViewController.php
@@ -49,7 +49,10 @@
     $targets = array();
     foreach ($build_targets as $build_target) {
       $header = id(new PHUIHeaderView())
-        ->setHeader(pht('Build Target %d', $build_target->getID()))
+        ->setHeader(pht(
+          'Build Target %d (%s)',
+          $build_target->getID(),
+          $build_target->getImplementation()->getName()))
         ->setUser($viewer);
       $properties = new PHUIPropertyListView();
 
@@ -280,7 +283,7 @@
       case HarbormasterBuild::STATUS_PENDING:
         return pht('Pending');
       case HarbormasterBuild::STATUS_WAITING:
-        return pht('Waiting on Resource');
+        return pht('Waiting');
       case HarbormasterBuild::STATUS_BUILDING:
         return pht('Building');
       case HarbormasterBuild::STATUS_PASSED:
Index: src/applications/harbormaster/controller/HarbormasterBuildableViewController.php
===================================================================
--- src/applications/harbormaster/controller/HarbormasterBuildableViewController.php
+++ src/applications/harbormaster/controller/HarbormasterBuildableViewController.php
@@ -52,8 +52,8 @@
             $item->addAttribute(pht('Pending'));
             break;
           case HarbormasterBuild::STATUS_WAITING:
-            $item->setBarColor('blue');
-            $item->addAttribute(pht('Waiting on Resource'));
+            $item->setBarColor('violet');
+            $item->addAttribute(pht('Waiting'));
             break;
           case HarbormasterBuild::STATUS_BUILDING:
             $item->setBarColor('yellow');
Index: src/applications/harbormaster/step/BuildStepImplementation.php
===================================================================
--- src/applications/harbormaster/step/BuildStepImplementation.php
+++ src/applications/harbormaster/step/BuildStepImplementation.php
@@ -53,7 +53,7 @@
   /**
    * Validate the current settings of this build step.
    */
-  public function validate() {
+  public function validateSettings() {
     return true;
   }
 
Index: src/applications/harbormaster/step/WaitForPreviousBuildStepImplementation.php
===================================================================
--- /dev/null
+++ src/applications/harbormaster/step/WaitForPreviousBuildStepImplementation.php
@@ -0,0 +1,127 @@
+<?php
+
+final class WaitForPreviousBuildStepImplementation
+  extends BuildStepImplementation {
+
+  public function getName() {
+    return pht('Wait for Previous Commits to Build');
+  }
+
+  public function getGenericDescription() {
+    return pht(
+      'Wait for previous commits to finish building the current plan '.
+      'before continuing.');
+  }
+
+  public function getDescription() {
+    return pht(
+      'Wait for previous commits to finish building the current plan '.
+      'before continuing.');
+  }
+
+  public function execute(
+    HarbormasterBuild $build,
+    HarbormasterBuildTarget $build_target) {
+
+    // We can only wait when building against commits.
+    $buildable = $build->getBuildable();
+    $object = $buildable->getBuildableObject();
+    if (!($object instanceof PhabricatorRepositoryCommit)) {
+      return;
+    }
+
+    // We are blocked until all previous builds finish.
+    $build->setBuildStatus(HarbormasterBuild::STATUS_WAITING);
+    $build->save();
+
+    // Block until all previous builds of the same build plan have
+    // finished.
+    $plan = $build->getBuildPlan();
+
+    $log = null;
+    $log_start = null;
+    $blockers = $this->getBlockers($object, $plan, $build);
+    while (count($blockers) > 0) {
+      if ($build->checkForCancellation()) {
+        if ($log !== null) {
+          $log->finalize($log_start);
+        }
+        return;
+      }
+
+      if ($log === null) {
+        $log = $build->createLog($build_target, "waiting", "blockers");
+        $log_start = $log->start();
+      }
+
+      $log->append("Blocked by: ".implode(",", $blockers)."\n");
+
+      sleep(1);
+      $blockers = $this->getBlockers($object, $plan, $build);
+    }
+    if ($log !== null) {
+      $log->finalize($log_start);
+    }
+
+    // Move back into building status.
+    $build->setBuildStatus(HarbormasterBuild::STATUS_BUILDING);
+    $build->save();
+  }
+
+  private function getBlockers(
+    PhabricatorRepositoryCommit $commit,
+    HarbormasterBuildPlan $plan,
+    HarbormasterBuild $source) {
+
+    $call = new ConduitCall(
+      'diffusion.commitparentsquery',
+      array(
+        'commit'   => $commit->getCommitIdentifier(),
+        'callsign' => $commit->getRepository()->getCallsign()
+      ));
+    $call->setUser(PhabricatorUser::getOmnipotentUser());
+    $parents = $call->execute();
+
+    $hashes = array();
+    foreach ($parents as $parent => $obj) {
+      $hashes[] = $parent;
+    }
+
+    $parents = id(new DiffusionCommitQuery())
+      ->setViewer(PhabricatorUser::getOmnipotentUser())
+      ->withRepository($commit->getRepository())
+      ->withIdentifiers($hashes)
+      ->execute();
+
+    $blockers = array();
+
+    $build_objects = array();
+    foreach ($parents as $parent) {
+      if (!$parent->isImported()) {
+        $blockers[] = pht('Commit %s', $parent->getCommitIdentifier());
+      } else {
+        $build_objects[] = $parent->getPHID();
+      }
+    }
+
+    $buildables = id(new HarbormasterBuildableQuery())
+      ->setViewer(PhabricatorUser::getOmnipotentUser())
+      ->withBuildablePHIDs($build_objects)
+      ->execute();
+    $buildable_phids = mpull($buildables, 'getPHID');
+
+    $builds = id(new HarbormasterBuildQuery())
+      ->setViewer(PhabricatorUser::getOmnipotentUser())
+      ->withBuildablePHIDs($buildable_phids)
+      ->withBuildPlanPHIDs(array($plan->getPHID()))
+      ->execute();
+
+    foreach ($builds as $build) {
+      if ($build->isBuilding()) {
+        $blockers[] = pht('Build %d', $build->getID());
+      }
+    }
+
+    return $blockers;
+  }
+}
Index: src/applications/harbormaster/storage/build/HarbormasterBuild.php
===================================================================
--- src/applications/harbormaster/storage/build/HarbormasterBuild.php
+++ src/applications/harbormaster/storage/build/HarbormasterBuild.php
@@ -94,6 +94,13 @@
     return $this->assertAttached($this->buildPlan);
   }
 
+  public function isBuilding() {
+    return $this->getBuildStatus() === self::STATUS_PENDING ||
+      $this->getBuildStatus() === self::STATUS_WAITING ||
+      $this->getBuildStatus() === self::STATUS_BUILDING ||
+      $this->getCancelRequested();
+  }
+
   public function createLog(
     HarbormasterBuildTarget $build_target,
     $log_source,