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
@@ -635,6 +635,7 @@
     'DiffusionCommitRevisionReviewersHeraldField' => 'applications/diffusion/herald/DiffusionCommitRevisionReviewersHeraldField.php',
     'DiffusionCommitRevisionSubscribersHeraldField' => 'applications/diffusion/herald/DiffusionCommitRevisionSubscribersHeraldField.php',
     'DiffusionCommitTagsController' => 'applications/diffusion/controller/DiffusionCommitTagsController.php',
+    'DiffusionCompareController' => 'applications/diffusion/controller/DiffusionCompareController.php',
     'DiffusionConduitAPIMethod' => 'applications/diffusion/conduit/DiffusionConduitAPIMethod.php',
     'DiffusionController' => 'applications/diffusion/controller/DiffusionController.php',
     'DiffusionCreateCommentConduitAPIMethod' => 'applications/diffusion/conduit/DiffusionCreateCommentConduitAPIMethod.php',
@@ -5273,6 +5274,7 @@
     'DiffusionCommitRevisionReviewersHeraldField' => 'DiffusionCommitHeraldField',
     'DiffusionCommitRevisionSubscribersHeraldField' => 'DiffusionCommitHeraldField',
     'DiffusionCommitTagsController' => 'DiffusionController',
+    'DiffusionCompareController' => 'DiffusionController',
     'DiffusionConduitAPIMethod' => 'ConduitAPIMethod',
     'DiffusionController' => 'PhabricatorController',
     'DiffusionCreateCommentConduitAPIMethod' => 'DiffusionConduitAPIMethod',
diff --git a/src/applications/diffusion/application/PhabricatorDiffusionApplication.php b/src/applications/diffusion/application/PhabricatorDiffusionApplication.php
--- a/src/applications/diffusion/application/PhabricatorDiffusionApplication.php
+++ b/src/applications/diffusion/application/PhabricatorDiffusionApplication.php
@@ -65,6 +65,7 @@
           => 'DiffusionCommitTagsController',
         'commit/(?P<commit>[a-z0-9]+)/edit/'
           => 'DiffusionCommitEditController',
+        'compare/' => 'DiffusionCompareController',
         'manage/(?:(?P<panel>[^/]+)/)?'
           => 'DiffusionRepositoryManagePanelsController',
         'uri/' => array(
diff --git a/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php b/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
--- a/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
+++ b/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
@@ -22,6 +22,7 @@
   protected function defineCustomParamTypes() {
     return array(
       'commit' => 'required string',
+      'against' => 'optional string',
       'path' => 'required string',
       'offset' => 'required int',
       'limit' => 'required int',
@@ -43,10 +44,17 @@
     $drequest = $this->getDiffusionRequest();
     $repository = $drequest->getRepository();
     $commit_hash = $request->getValue('commit');
+    $against_hash = $request->getValue('against');
     $path = $request->getValue('path');
     $offset = $request->getValue('offset');
     $limit = $request->getValue('limit');
 
+    if (strlen($against_hash)) {
+      $commit_range = "${against_hash}..${commit_hash}";
+    } else {
+      $commit_range = $commit_hash;
+    }
+
     list($stdout) = $repository->execxLocalCommand(
       'log '.
         '--skip=%d '.
@@ -56,15 +64,12 @@
       $offset,
       $limit,
       '%H:%P',
-      $commit_hash,
+      $commit_range,
       // Git omits merge commits if the path is provided, even if it is empty.
       (strlen($path) ? csprintf('%s', $path) : ''));
 
     $lines = explode("\n", trim($stdout));
     $lines = array_filter($lines);
-    if (!$lines) {
-      return array();
-    }
 
     $hash_list = array();
     $parent_map = array();
@@ -76,6 +81,10 @@
 
     $this->parents = $parent_map;
 
+    if (!$hash_list) {
+      return array();
+    }
+
     return DiffusionQuery::loadHistoryForCommitIdentifiers(
       $hash_list,
       $drequest);
diff --git a/src/applications/diffusion/controller/DiffusionCompareController.php b/src/applications/diffusion/controller/DiffusionCompareController.php
new file mode 100644
--- /dev/null
+++ b/src/applications/diffusion/controller/DiffusionCompareController.php
@@ -0,0 +1,260 @@
+<?php
+
+final class DiffusionCompareController extends DiffusionController {
+
+  public function shouldAllowPublic() {
+    return true;
+  }
+
+  public function handleRequest(AphrontRequest $request) {
+    $response = $this->loadDiffusionContext();
+    if ($response) {
+      return $response;
+    }
+
+    $viewer = $this->getViewer();
+    $drequest = $this->getDiffusionRequest();
+    $repository = $drequest->getRepository();
+
+    $content = array();
+
+    $crumbs = $this->buildCrumbs(array('view' => 'compare'));
+
+    $empty_title = null;
+    $error_message = null;
+
+    if ($repository->getVersionControlSystem() !=
+          PhabricatorRepositoryType::REPOSITORY_TYPE_GIT) {
+      $empty_title = pht('Not supported');
+      $error_message = pht(
+        'This feature is not yet supported for %s repositories.',
+        $repository->getVersionControlSystem());
+      $content[] = id(new PHUIInfoView())
+        ->setTitle($empty_title)
+        ->setSeverity(PHUIInfoView::SEVERITY_WARNING)
+        ->setErrors(array($error_message));
+    }
+
+    $head_ref = $request->getStr('head');
+    $against_ref = $request->getStr('against');
+
+    $refs = id(new DiffusionCachedResolveRefsQuery())
+      ->setRepository($repository)
+      ->withRefs(array($head_ref, $against_ref))
+      ->execute();
+
+
+    if (count($refs) == 2) {
+      $content[] = $this->buildCompareContent($drequest);
+    } else {
+      $content[] = $this->buildCompareForm($request, $refs);
+    }
+
+
+    return $this->newPage()
+      ->setTitle(
+        array(
+          $repository->getName(),
+          $repository->getDisplayName(),
+        ))
+      ->setCrumbs($crumbs)
+      ->appendChild($content);
+  }
+
+  private function buildCompareForm(AphrontRequest $request, array $resolved) {
+    $viewer = $this->getViewer();
+
+    $head_ref = $request->getStr('head');
+    $against_ref = $request->getStr('against');
+
+    if (idx($resolved, $head_ref)) {
+      $e_head = null;
+    } else {
+      $e_head = 'Not Found';
+    }
+
+    if (idx($resolved, $against_ref)) {
+      $e_against = null;
+    } else {
+      $e_against = 'Not Found';
+    }
+
+
+    $form = id(new AphrontFormView())
+      ->setUser($viewer)
+      ->setMethod('GET')
+      ->appendControl(
+        id(new AphrontFormTextControl())
+          ->setLabel(pht('Head'))
+          ->setName('head')
+          ->setError($e_head)
+          ->setValue($head_ref))
+      ->appendControl(
+        id(new AphrontFormTextControl())
+          ->setLabel(pht('Against'))
+          ->setName('against')
+          ->setError($e_against)
+          ->setValue($against_ref))
+      ->appendControl(
+        id(new AphrontFormSubmitControl())
+          ->setValue('Compare'));
+
+    return $form;
+  }
+
+  private function buildCompareContent(DiffusionRequest $drequest) {
+    $request = $this->getRequest();
+    $repository = $drequest->getRepository();
+
+    $head_ref = $request->getStr('head');
+    $against_ref = $request->getStr('against');
+
+    $content = array();
+
+    $pager = id(new PHUIPagerView())
+      ->readFromRequest($request);
+
+    try {
+      $history_results = $this->callConduitWithDiffusionRequest(
+        'diffusion.historyquery',
+        array(
+          'commit' => $head_ref,
+          'against' => $against_ref,
+          'path' => $drequest->getPath(),
+          'offset' => $pager->getOffset(),
+          'limit' => $pager->getPageSize() + 1,
+        ));
+      $history = DiffusionPathChange::newFromConduit(
+        $history_results['pathChanges']);
+
+      $history = $pager->sliceResults($history);
+
+      $history_exception = null;
+    } catch (Exception $ex) {
+      $history_results = null;
+      $history = null;
+      $history_exception = $ex;
+    }
+
+    if ($history_results) {
+        $content[] = $this->buildCompareProperties($drequest, $history_results);
+    }
+    $content[] = $this->buildCompareForm(
+      $request,
+      array($head_ref => true, $against_ref => true));
+
+    $content[] = $this->buildHistoryTable(
+      $history_results,
+      $history,
+      $pager,
+      $history_exception);
+
+    $content[] = $this->renderTablePagerBox($pager);
+
+    return $content;
+  }
+
+  private function buildCompareProperties($drequest, array $history_results) {
+    $viewer = $this->getViewer();
+
+    $request = $this->getRequest();
+    $repository = $drequest->getRepository();
+
+    $head_ref = $request->getStr('head');
+    $against_ref = $request->getStr('against');
+
+    $reverse_uri = $repository->getPathURI(
+      "compare/?head=${against_ref}&against=${head_ref}");
+    $actions = id(new PhabricatorActionListView());
+    $actions->setUser($viewer);
+
+
+    $actions->addAction(id(new PhabricatorActionView())
+        ->setName(pht('Reverse Comparison'))
+        ->setHref($reverse_uri)
+        ->setIcon('fa-list'));
+
+    $view = id(new PHUIPropertyListView())
+      ->setUser($viewer)
+      ->setActionList($actions);
+
+    $readme =
+      'These are the commits that are reachable from **Head** commit and not '.
+      'from the **Against** commit.';
+    $readme = new PHUIRemarkupView($viewer, $readme);
+    $view->addTextContent($readme);
+
+    $view->addProperty(pht('Head'), $head_ref);
+    $view->addProperty(pht('Against'), $against_ref);
+
+    $header = id(new PHUIHeaderView())
+      ->setUser($viewer)
+      ->setPolicyObject($drequest->getRepository());
+
+    $box = id(new PHUIObjectBoxView())
+      ->setUser($viewer)
+      ->setHeader($header)
+      ->addPropertyList($view);
+    return $box;
+  }
+
+  private function buildHistoryTable(
+    $history_results,
+    $history,
+    $pager,
+    $history_exception) {
+
+    $request = $this->getRequest();
+    $viewer = $request->getUser();
+    $drequest = $this->getDiffusionRequest();
+    $repository = $drequest->getRepository();
+
+    if ($history_exception) {
+      if ($repository->isImporting()) {
+        return $this->renderStatusMessage(
+          pht('Still Importing...'),
+          pht(
+            'This repository is still importing. History is not yet '.
+            'available.'));
+      } else {
+        return $this->renderStatusMessage(
+          pht('Unable to Retrieve History'),
+          $history_exception->getMessage());
+      }
+    }
+
+    if (!$history) {
+      return $this->renderStatusMessage(
+        pht('Up to date'),
+        new PHUIRemarkupView(
+          $viewer,
+          pht(
+            '**Against** is strictly ahead of **Head** '.
+            '- no commits are missing.')));
+    }
+
+    $history_table = id(new DiffusionHistoryTableView())
+      ->setUser($viewer)
+      ->setDiffusionRequest($drequest)
+      ->setHistory($history);
+
+    // TODO: Super sketchy?
+    $history_table->loadRevisions();
+
+    if ($history_results) {
+      $history_table->setParents($history_results['parents']);
+      $history_table->setIsHead(!$pager->getOffset());
+      $history_table->setIsTail(!$pager->getHasMorePages());
+    }
+
+    // TODO also expose diff.
+
+    $panel = new PHUIObjectBoxView();
+    $header = id(new PHUIHeaderView())
+      ->setHeader(pht('Missing Commits'));
+    $panel->setHeader($header);
+    $panel->setTable($history_table);
+
+    return $panel;
+  }
+}
diff --git a/src/applications/diffusion/controller/DiffusionController.php b/src/applications/diffusion/controller/DiffusionController.php
--- a/src/applications/diffusion/controller/DiffusionController.php
+++ b/src/applications/diffusion/controller/DiffusionController.php
@@ -213,6 +213,9 @@
       case 'change':
         $view_name = pht('Change');
         break;
+      case 'compare':
+        $view_name = pht('Compare');
+        break;
     }
 
     $crumb = id(new PHUICrumbView())