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
@@ -1263,6 +1263,7 @@
     'FundInitiativeTransactionType' => 'applications/fund/xaction/FundInitiativeTransactionType.php',
     'FundInitiativeViewController' => 'applications/fund/controller/FundInitiativeViewController.php',
     'FundSchemaSpec' => 'applications/fund/storage/FundSchemaSpec.php',
+    'HarbormasterAbortOlderBuildsBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterAbortOlderBuildsBuildStepImplementation.php',
     'HarbormasterArcLintBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterArcLintBuildStepImplementation.php',
     'HarbormasterArcUnitBuildStepImplementation' => 'applications/harbormaster/step/HarbormasterArcUnitBuildStepImplementation.php',
     'HarbormasterArtifact' => 'applications/harbormaster/artifact/HarbormasterArtifact.php',
@@ -1358,6 +1359,7 @@
     'HarbormasterCircleCIBuildableInterface' => 'applications/harbormaster/interface/HarbormasterCircleCIBuildableInterface.php',
     'HarbormasterCircleCIHookController' => 'applications/harbormaster/controller/HarbormasterCircleCIHookController.php',
     'HarbormasterConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterConduitAPIMethod.php',
+    'HarbormasterControlBuildStepGroup' => 'applications/harbormaster/stepgroup/HarbormasterControlBuildStepGroup.php',
     'HarbormasterController' => 'applications/harbormaster/controller/HarbormasterController.php',
     'HarbormasterCreateArtifactConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterCreateArtifactConduitAPIMethod.php',
     'HarbormasterCreatePlansCapability' => 'applications/harbormaster/capability/HarbormasterCreatePlansCapability.php',
@@ -6636,6 +6638,7 @@
     'FundInitiativeTransactionType' => 'PhabricatorModularTransactionType',
     'FundInitiativeViewController' => 'FundController',
     'FundSchemaSpec' => 'PhabricatorConfigSchemaSpec',
+    'HarbormasterAbortOlderBuildsBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterArcLintBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterArcUnitBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterArtifact' => 'Phobject',
@@ -6771,6 +6774,7 @@
     'HarbormasterCircleCIBuildStepImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterCircleCIHookController' => 'HarbormasterController',
     'HarbormasterConduitAPIMethod' => 'ConduitAPIMethod',
+    'HarbormasterControlBuildStepGroup' => 'HarbormasterBuildStepGroup',
     'HarbormasterController' => 'PhabricatorController',
     'HarbormasterCreateArtifactConduitAPIMethod' => 'HarbormasterConduitAPIMethod',
     'HarbormasterCreatePlansCapability' => 'PhabricatorPolicyCapability',
diff --git a/src/applications/harbormaster/constants/HarbormasterBuildStatus.php b/src/applications/harbormaster/constants/HarbormasterBuildStatus.php
--- a/src/applications/harbormaster/constants/HarbormasterBuildStatus.php
+++ b/src/applications/harbormaster/constants/HarbormasterBuildStatus.php
@@ -98,6 +98,19 @@
     );
   }
 
+  public static function getIncompleteStatusConstants() {
+    $map = self::getBuildStatusSpecMap();
+
+    $constants = array();
+    foreach ($map as $constant => $spec) {
+      if (!$spec['isComplete']) {
+        $constants[] = $constant;
+      }
+    }
+
+    return $constants;
+  }
+
   public static function getCompletedStatusConstants() {
     return array(
       self::STATUS_PASSED,
diff --git a/src/applications/harbormaster/controller/HarbormasterBuildActionController.php b/src/applications/harbormaster/controller/HarbormasterBuildActionController.php
--- a/src/applications/harbormaster/controller/HarbormasterBuildActionController.php
+++ b/src/applications/harbormaster/controller/HarbormasterBuildActionController.php
@@ -51,18 +51,7 @@
     }
 
     if ($request->isDialogFormPost() && $can_issue) {
-      $editor = id(new HarbormasterBuildTransactionEditor())
-        ->setActor($viewer)
-        ->setContentSourceFromRequest($request)
-        ->setContinueOnNoEffect(true)
-        ->setContinueOnMissingFields(true);
-
-      $xaction = id(new HarbormasterBuildTransaction())
-        ->setTransactionType(HarbormasterBuildTransaction::TYPE_COMMAND)
-        ->setNewValue($action);
-
-      $editor->applyTransactions($build, array($xaction));
-
+      $build->sendMessage($viewer, $action);
       return id(new AphrontRedirectResponse())->setURI($return_uri);
     }
 
diff --git a/src/applications/harbormaster/query/HarbormasterBuildStepQuery.php b/src/applications/harbormaster/query/HarbormasterBuildStepQuery.php
--- a/src/applications/harbormaster/query/HarbormasterBuildStepQuery.php
+++ b/src/applications/harbormaster/query/HarbormasterBuildStepQuery.php
@@ -22,48 +22,39 @@
     return $this;
   }
 
+  public function newResultObject() {
+    return new HarbormasterBuildStep();
+  }
+
   protected function loadPage() {
-    $table = new HarbormasterBuildStep();
-    $conn_r = $table->establishConnection('r');
-
-    $data = queryfx_all(
-      $conn_r,
-      'SELECT * FROM %T %Q %Q %Q',
-      $table->getTableName(),
-      $this->buildWhereClause($conn_r),
-      $this->buildOrderClause($conn_r),
-      $this->buildLimitClause($conn_r));
-
-    return $table->loadAllFromArray($data);
+    return $this->loadStandardPage($this->newResultObject());
   }
 
-  protected function buildWhereClause(AphrontDatabaseConnection $conn_r) {
-    $where = array();
+  protected function buildWhereClauseParts(AphrontDatabaseConnection $conn) {
+    $where = parent::buildWhereClauseParts($conn);
 
-    if ($this->ids) {
+    if ($this->ids !== null) {
       $where[] = qsprintf(
-        $conn_r,
+        $conn,
         'id IN (%Ld)',
         $this->ids);
     }
 
-    if ($this->phids) {
+    if ($this->phids !== null) {
       $where[] = qsprintf(
-        $conn_r,
+        $conn,
         'phid in (%Ls)',
         $this->phids);
     }
 
-    if ($this->buildPlanPHIDs) {
+    if ($this->buildPlanPHIDs !== null) {
       $where[] = qsprintf(
-        $conn_r,
+        $conn,
         'buildPlanPHID in (%Ls)',
         $this->buildPlanPHIDs);
     }
 
-    $where[] = $this->buildPagingClause($conn_r);
-
-    return $this->formatWhereClause($where);
+    return $where;
   }
 
   protected function willFilterPage(array $page) {
diff --git a/src/applications/harbormaster/step/HarbormasterAbortOlderBuildsBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterAbortOlderBuildsBuildStepImplementation.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/step/HarbormasterAbortOlderBuildsBuildStepImplementation.php
@@ -0,0 +1,135 @@
+<?php
+
+final class HarbormasterAbortOlderBuildsBuildStepImplementation
+  extends HarbormasterBuildStepImplementation {
+
+  public function getName() {
+    return pht('Abort Older Builds');
+  }
+
+  public function getGenericDescription() {
+    return pht(
+      'When building a revision, abort copies of this build plan which are '.
+      'currently running against older diffs.');
+  }
+
+  public function getBuildStepGroupKey() {
+    return HarbormasterControlBuildStepGroup::GROUPKEY;
+  }
+
+  public function getEditInstructions() {
+    return pht(<<<EOTEXT
+When run against a revision, this build step will abort any older copies of
+the same build plan which are currently running against older diffs.
+
+There are some nuances to the behavior:
+
+  - if this build step is triggered manually, it won't abort anything;
+  - this build step won't abort manual builds;
+  - this build step won't abort anything if the diff it is building isn't
+    the active diff when it runs.
+
+Build results on outdated diffs often aren't very important, so this may
+reduce build queue load without any substantial cost.
+EOTEXT
+      );
+  }
+
+  public function willStartBuild(
+    PhabricatorUser $viewer,
+    HarbormasterBuildable $buildable,
+    HarbormasterBuild $build,
+    HarbormasterBuildPlan $plan,
+    HarbormasterBuildStep $step) {
+
+    if ($buildable->getIsManualBuildable()) {
+      // Don't abort anything if this is a manual buildable.
+      return;
+    }
+
+    $object_phid = $buildable->getBuildablePHID();
+    if (phid_get_type($object_phid) !== DifferentialDiffPHIDType::TYPECONST) {
+      // If this buildable isn't building a diff, bail out. For example, we
+      // might be building a commit. In this case, this step has no effect.
+      return;
+    }
+
+    $diff = id(new DifferentialDiffQuery())
+      ->setViewer($viewer)
+      ->withPHIDs(array($object_phid))
+      ->executeOne();
+    if (!$diff) {
+      return;
+    }
+
+    $revision_id = $diff->getRevisionID();
+
+    $revision = id(new DifferentialRevisionQuery())
+      ->setViewer($viewer)
+      ->withIDs(array($revision_id))
+      ->executeOne();
+    if (!$revision) {
+      return;
+    }
+
+    $active_phid = $revision->getActiveDiffPHID();
+    if ($active_phid !== $object_phid) {
+      // If we aren't building the active diff, bail out.
+      return;
+    }
+
+    $diffs = id(new DifferentialDiffQuery())
+      ->setViewer($viewer)
+      ->withRevisionIDs(array($revision_id))
+      ->execute();
+    $abort_diff_phids = array();
+    foreach ($diffs as $diff) {
+      if ($diff->getPHID() !== $active_phid) {
+        $abort_diff_phids[] = $diff->getPHID();
+      }
+    }
+
+    if (!$abort_diff_phids) {
+      return;
+    }
+
+    // We're fetching buildables even if they have "passed" or "failed"
+    // because they may still have ongoing builds. At the time of writing
+    // only "failed" buildables may still be ongoing, but it seems likely that
+    // "passed" buildables may be ongoing in the future.
+
+    $abort_buildables = id(new HarbormasterBuildableQuery())
+      ->setViewer($viewer)
+      ->withBuildablePHIDs($abort_diff_phids)
+      ->withManualBuildables(false)
+      ->execute();
+    if (!$abort_buildables) {
+      return;
+    }
+
+    $statuses = HarbormasterBuildStatus::getIncompleteStatusConstants();
+
+    $abort_builds = id(new HarbormasterBuildQuery())
+      ->setViewer($viewer)
+      ->withBuildablePHIDs(mpull($abort_buildables, 'getPHID'))
+      ->withBuildPlanPHIDs(array($plan->getPHID()))
+      ->withBuildStatuses($statuses)
+      ->execute();
+    if (!$abort_builds) {
+      return;
+    }
+
+    foreach ($abort_builds as $abort_build) {
+      $abort_build->sendMessage(
+        $viewer,
+        HarbormasterBuildCommand::COMMAND_ABORT);
+    }
+  }
+
+  public function execute(
+    HarbormasterBuild $build,
+    HarbormasterBuildTarget $build_target) {
+    return;
+  }
+
+}
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
@@ -308,6 +308,15 @@
           'enabled in configuration.'));
   }
 
+  public function willStartBuild(
+    PhabricatorUser $viewer,
+    HarbormasterBuildable $buildable,
+    HarbormasterBuild $build,
+    HarbormasterBuildPlan $plan,
+    HarbormasterBuildStep $step) {
+    return;
+  }
+
 
 /* -(  Automatic Targets  )-------------------------------------------------- */
 
diff --git a/src/applications/harbormaster/stepgroup/HarbormasterControlBuildStepGroup.php b/src/applications/harbormaster/stepgroup/HarbormasterControlBuildStepGroup.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/stepgroup/HarbormasterControlBuildStepGroup.php
@@ -0,0 +1,20 @@
+<?php
+
+final class HarbormasterControlBuildStepGroup
+  extends HarbormasterBuildStepGroup {
+
+  const GROUPKEY = 'harbormaster.control';
+
+  public function getGroupName() {
+    return pht('Flow Control');
+  }
+
+  public function getGroupOrder() {
+    return 5000;
+  }
+
+  public function shouldShowIfEmpty() {
+    return false;
+  }
+
+}
diff --git a/src/applications/harbormaster/storage/HarbormasterBuildable.php b/src/applications/harbormaster/storage/HarbormasterBuildable.php
--- a/src/applications/harbormaster/storage/HarbormasterBuildable.php
+++ b/src/applications/harbormaster/storage/HarbormasterBuildable.php
@@ -141,6 +141,15 @@
 
     $build->save();
 
+    $steps = id(new HarbormasterBuildStepQuery())
+      ->setViewer($viewer)
+      ->withBuildPlanPHIDs(array($plan->getPHID()))
+      ->execute();
+
+    foreach ($steps as $step) {
+      $step->willStartBuild($viewer, $this, $build, $plan);
+    }
+
     PhabricatorWorker::scheduleTask(
       'HarbormasterBuildWorker',
       array(
diff --git a/src/applications/harbormaster/storage/build/HarbormasterBuild.php b/src/applications/harbormaster/storage/build/HarbormasterBuild.php
--- a/src/applications/harbormaster/storage/build/HarbormasterBuild.php
+++ b/src/applications/harbormaster/storage/build/HarbormasterBuild.php
@@ -356,6 +356,35 @@
     }
   }
 
+  public function sendMessage(PhabricatorUser $viewer, $command) {
+    // TODO: This should not be an editor transaction, but there are plans to
+    // merge BuildCommand into BuildMessage which should moot this. As this
+    // exists today, it can race against BuildEngine.
+
+    // This is a bogus content source, but this whole flow should be obsolete
+    // soon.
+    $content_source = PhabricatorContentSource::newForSource(
+      PhabricatorConsoleContentSource::SOURCECONST);
+
+    $editor = id(new HarbormasterBuildTransactionEditor())
+      ->setActor($viewer)
+      ->setContentSource($content_source)
+      ->setContinueOnNoEffect(true)
+      ->setContinueOnMissingFields(true);
+
+    $viewer_phid = $viewer->getPHID();
+    if (!$viewer_phid) {
+      $acting_phid = id(new PhabricatorHarbormasterApplication())->getPHID();
+      $editor->setActingAsPHID($acting_phid);
+    }
+
+    $xaction = id(new HarbormasterBuildTransaction())
+      ->setTransactionType(HarbormasterBuildTransaction::TYPE_COMMAND)
+      ->setNewValue($command);
+
+    $editor->applyTransactions($this, array($xaction));
+  }
+
 
 /* -(  PhabricatorApplicationTransactionInterface  )------------------------- */
 
diff --git a/src/applications/harbormaster/storage/configuration/HarbormasterBuildStep.php b/src/applications/harbormaster/storage/configuration/HarbormasterBuildStep.php
--- a/src/applications/harbormaster/storage/configuration/HarbormasterBuildStep.php
+++ b/src/applications/harbormaster/storage/configuration/HarbormasterBuildStep.php
@@ -100,6 +100,19 @@
     return ($this->getStepAutoKey() !== null);
   }
 
+  public function willStartBuild(
+    PhabricatorUser $viewer,
+    HarbormasterBuildable $buildable,
+    HarbormasterBuild $build,
+    HarbormasterBuildPlan $plan) {
+    return $this->getStepImplementation()->willStartBuild(
+      $viewer,
+      $buildable,
+      $build,
+      $plan,
+      $this);
+  }
+
 
 /* -(  PhabricatorApplicationTransactionInterface  )------------------------- */