diff --git a/resources/sql/autopatches/20140821.harbormasterbuildgen.1.sql b/resources/sql/autopatches/20140821.harbormasterbuildgen.1.sql
new file mode 100644
--- /dev/null
+++ b/resources/sql/autopatches/20140821.harbormasterbuildgen.1.sql
@@ -0,0 +1,5 @@
+ALTER TABLE {$NAMESPACE}_harbormaster.harbormaster_build
+  ADD COLUMN `buildGeneration` INT UNSIGNED NOT NULL DEFAULT 0;
+
+ALTER TABLE {$NAMESPACE}_harbormaster.harbormaster_buildtarget
+  ADD COLUMN `buildGeneration` INT UNSIGNED NOT NULL DEFAULT 0;
diff --git a/src/applications/harbormaster/controller/HarbormasterBuildViewController.php b/src/applications/harbormaster/controller/HarbormasterBuildViewController.php
--- a/src/applications/harbormaster/controller/HarbormasterBuildViewController.php
+++ b/src/applications/harbormaster/controller/HarbormasterBuildViewController.php
@@ -14,6 +14,7 @@
     $viewer = $request->getUser();
 
     $id = $this->id;
+    $generation = $request->getInt('g');
 
     $build = id(new HarbormasterBuildQuery())
       ->setViewer($viewer)
@@ -52,13 +53,18 @@
       '/'.$build->getBuildable()->getMonogram());
     $crumbs->addTextCrumb($title);
 
+    if ($generation === null || $generation > $build->getBuildGeneration() ||
+      $generation < 0) {
+      $generation = $build->getBuildGeneration();
+    }
+
     $build_targets = id(new HarbormasterBuildTargetQuery())
       ->setViewer($viewer)
       ->needBuildSteps(true)
       ->withBuildPHIDs(array($build->getPHID()))
+      ->withBuildGenerations(array($generation))
       ->execute();
 
-
     if ($build_targets) {
       $messages = id(new HarbormasterBuildMessageQuery())
         ->setViewer($viewer)
@@ -435,9 +441,12 @@
       $handles[$build->getBuildPlanPHID()]->renderLink());
 
     $properties->addProperty(
+      pht('Restarts'),
+      $build->getBuildGeneration());
+
+    $properties->addProperty(
       pht('Status'),
       $this->getStatus($build));
-
   }
 
   private function getStatus(HarbormasterBuild $build) {
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
@@ -100,7 +100,7 @@
   private function updateBuild(HarbormasterBuild $build) {
     if (($build->getBuildStatus() == HarbormasterBuild::STATUS_PENDING) ||
         ($build->isRestarting())) {
-      $this->destroyBuildTargets($build);
+      $this->restartBuild($build);
       $build->setBuildStatus(HarbormasterBuild::STATUS_BUILDING);
       $build->save();
     }
@@ -122,38 +122,28 @@
     }
   }
 
-  private function destroyBuildTargets(HarbormasterBuild $build) {
-    $this->releaseAllArtifacts($build);
-
-    $targets = id(new HarbormasterBuildTargetQuery())
-      ->setViewer($this->getViewer())
-      ->withBuildPHIDs(array($build->getPHID()))
-      ->execute();
+  private function restartBuild(HarbormasterBuild $build) {
 
-    if (!$targets) {
-      return;
-    }
-
-    $target_phids = mpull($targets, 'getPHID');
+    // We're restarting the build, so release all previous artifacts.
+    $this->releaseAllArtifacts($build);
 
-    $artifacts = id(new HarbormasterBuildArtifactQuery())
-      ->setViewer($this->getViewer())
-      ->withBuildTargetPHIDs($target_phids)
-      ->execute();
+    // Increment the build generation counter on the build.
+    $build->setBuildGeneration($build->getBuildGeneration() + 1);
 
-    foreach ($artifacts as $artifact) {
-      $artifact->delete();
-    }
+    // TODO: Currently running targets should periodically check their build
+    // generation (which won't have changed) against the build's generation.
+    // If it is different, they should automatically stop what they're doing
+    // and abort.
 
-    foreach ($targets as $target) {
-      $target->delete();
-    }
+    // Previously we used to delete targets, logs and artifacts here.  Instead
+    // leave them around so users can view previous generations of this build.
   }
 
   private function updateBuildSteps(HarbormasterBuild $build) {
     $targets = id(new HarbormasterBuildTargetQuery())
       ->setViewer($this->getViewer())
       ->withBuildPHIDs(array($build->getPHID()))
+      ->withBuildGenerations(array($build->getBuildGeneration()))
       ->execute();
 
     $this->updateWaitingTargets($targets);
@@ -454,6 +444,7 @@
     $targets = id(new HarbormasterBuildTargetQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withBuildPHIDs(array($build->getPHID()))
+      ->withBuildGenerations(array($build->getBuildGeneration()))
       ->execute();
 
     if (count($targets) === 0) {
diff --git a/src/applications/harbormaster/query/HarbormasterBuildQuery.php b/src/applications/harbormaster/query/HarbormasterBuildQuery.php
--- a/src/applications/harbormaster/query/HarbormasterBuildQuery.php
+++ b/src/applications/harbormaster/query/HarbormasterBuildQuery.php
@@ -122,6 +122,13 @@
       $targets = mgroup($targets, 'getBuildPHID');
       foreach ($page as $build) {
         $build_targets = idx($targets, $build->getPHID(), array());
+
+        foreach ($build_targets as $phid => $target) {
+          if ($target->getBuildGeneration() !== $build->getBuildGeneration()) {
+            unset($build_targets[$phid]);
+          }
+        }
+
         $build->attachBuildTargets($build_targets);
       }
     }
diff --git a/src/applications/harbormaster/query/HarbormasterBuildTargetQuery.php b/src/applications/harbormaster/query/HarbormasterBuildTargetQuery.php
--- a/src/applications/harbormaster/query/HarbormasterBuildTargetQuery.php
+++ b/src/applications/harbormaster/query/HarbormasterBuildTargetQuery.php
@@ -6,6 +6,7 @@
   private $ids;
   private $phids;
   private $buildPHIDs;
+  private $buildGenerations;
   private $needBuildSteps;
 
   public function withIDs(array $ids) {
@@ -23,6 +24,11 @@
     return $this;
   }
 
+  public function withBuildGenerations(array $build_generations) {
+    $this->buildGenerations = $build_generations;
+    return $this;
+  }
+
   public function needBuildSteps($need_build_steps) {
     $this->needBuildSteps = $need_build_steps;
     return $this;
@@ -67,6 +73,13 @@
         $this->buildPHIDs);
     }
 
+    if ($this->buildGenerations) {
+      $where[] = qsprintf(
+        $conn_r,
+        'buildGeneration in (%Ld)',
+        $this->buildGenerations);
+    }
+
     $where[] = $this->buildPagingClause($conn_r);
 
     return $this->formatWhereClause($where);
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
@@ -6,6 +6,7 @@
   protected $buildablePHID;
   protected $buildPlanPHID;
   protected $buildStatus;
+  protected $buildGeneration;
 
   private $buildable = self::ATTACHABLE;
   private $buildPlan = self::ATTACHABLE;
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
@@ -12,6 +12,7 @@
   protected $targetStatus;
   protected $dateStarted;
   protected $dateCompleted;
+  protected $buildGeneration;
 
   const STATUS_PENDING = 'target/pending';
   const STATUS_BUILDING = 'target/building';
@@ -82,7 +83,8 @@
       ->setClassName($build_step->getClassName())
       ->setDetails($build_step->getDetails())
       ->setTargetStatus(self::STATUS_PENDING)
-      ->setVariables($variables);
+      ->setVariables($variables)
+      ->setBuildGeneration($build->getBuildGeneration());
   }
 
   public function getConfiguration() {