diff --git a/resources/sql/autopatches/20180809.repo_identities.activity.php b/resources/sql/autopatches/20180809.repo_identities.activity.php
new file mode 100644
index 0000000000..e1077d4ddb
--- /dev/null
+++ b/resources/sql/autopatches/20180809.repo_identities.activity.php
@@ -0,0 +1,20 @@
+<?php
+
+// Advise installs to rebuild the repository identities.
+
+// If the install has no commits (or no commits that lack an
+// authorIdentityPHID), don't require a rebuild.
+$commits = id(new PhabricatorRepositoryCommit())
+  ->loadAllWhere('authorIdentityPHID IS NULL LIMIT 1');
+
+if (!$commits) {
+  return;
+}
+
+try {
+  id(new PhabricatorConfigManualActivity())
+    ->setActivityType(PhabricatorConfigManualActivity::TYPE_IDENTITIES)
+    ->save();
+} catch (AphrontDuplicateKeyQueryException $ex) {
+  // If we've already noted that this activity is required, just move on.
+}
diff --git a/src/applications/config/check/PhabricatorManualActivitySetupCheck.php b/src/applications/config/check/PhabricatorManualActivitySetupCheck.php
index 44db1b57a6..c5c756e49b 100644
--- a/src/applications/config/check/PhabricatorManualActivitySetupCheck.php
+++ b/src/applications/config/check/PhabricatorManualActivitySetupCheck.php
@@ -1,75 +1,146 @@
 <?php
 
 final class PhabricatorManualActivitySetupCheck
   extends PhabricatorSetupCheck {
 
   public function getDefaultGroup() {
     return self::GROUP_OTHER;
   }
 
   protected function executeChecks() {
     $activities = id(new PhabricatorConfigManualActivity())->loadAll();
 
     foreach ($activities as $activity) {
       $type = $activity->getActivityType();
 
-      // For now, there is only one type of manual activity. It's not clear
-      // if we're really going to have too much more of this stuff so this
-      // is a bit under-designed for now.
-
-      $activity_name = pht('Rebuild Search Index');
-      $activity_summary = pht(
-        'The search index algorithm has been updated and the index needs '.
-        'be rebuilt.');
-
-      $message = array();
-
-      $message[] = pht(
-        'The indexing algorithm for the fulltext search index has been '.
-        'updated and the index needs to be rebuilt. Until you rebuild the '.
-        'index, global search (and other fulltext search) will not '.
-        'function correctly.');
-
-      $message[] = pht(
-        'You can rebuild the search index while Phabricator is running.');
-
-      $message[] = pht(
-        'To rebuild the index, run this command:');
-
-      $message[] = phutil_tag(
-        'pre',
-        array(),
-        (string)csprintf(
-          'phabricator/ $ ./bin/search index --all --force --background'));
-
-      $message[] = pht(
-        'You can find more information about rebuilding the search '.
-        'index here: %s',
-        phutil_tag(
-          'a',
-          array(
-            'href' => 'https://phurl.io/u/reindex',
-            'target' => '_blank',
-          ),
-          'https://phurl.io/u/reindex'));
-
-      $message[] = pht(
-        'After rebuilding the index, run this command to clear this setup '.
-        'warning:');
-
-      $message[] = phutil_tag(
-        'pre',
-        array(),
-        (string)csprintf('phabricator/ $ ./bin/config done %R', $type));
-
-      $activity_message = phutil_implode_html("\n\n", $message);
-
-      $this->newIssue('manual.'.$type)
-        ->setName($activity_name)
-        ->setSummary($activity_summary)
-        ->setMessage($activity_message);
+      switch ($type) {
+        case PhabricatorConfigManualActivity::TYPE_REINDEX:
+          $this->raiseSearchReindexIssue();
+          break;
+
+        case PhabricatorConfigManualActivity::TYPE_IDENTITIES:
+          $this->raiseRebuildIdentitiesIssue();
+          break;
+
+        default:
+      }
     }
+  }
+
+  private function raiseSearchReindexIssue() {
+    $activity_name = pht('Rebuild Search Index');
+    $activity_summary = pht(
+      'The search index algorithm has been updated and the index needs '.
+      'be rebuilt.');
+
+    $message = array();
+
+    $message[] = pht(
+      'The indexing algorithm for the fulltext search index has been '.
+      'updated and the index needs to be rebuilt. Until you rebuild the '.
+      'index, global search (and other fulltext search) will not '.
+      'function correctly.');
+
+    $message[] = pht(
+      'You can rebuild the search index while Phabricator is running.');
+
+    $message[] = pht(
+      'To rebuild the index, run this command:');
+
+    $message[] = phutil_tag(
+      'pre',
+      array(),
+      (string)csprintf(
+        'phabricator/ $ ./bin/search index --all --force --background'));
+
+    $message[] = pht(
+      'You can find more information about rebuilding the search '.
+      'index here: %s',
+      phutil_tag(
+        'a',
+        array(
+          'href' => 'https://phurl.io/u/reindex',
+          'target' => '_blank',
+        ),
+        'https://phurl.io/u/reindex'));
+
+    $message[] = pht(
+      'After rebuilding the index, run this command to clear this setup '.
+      'warning:');
+
+    $message[] = phutil_tag(
+      'pre',
+      array(),
+      'phabricator/ $ ./bin/config done reindex');
+
+    $activity_message = phutil_implode_html("\n\n", $message);
+
+    $this->newIssue('manual.reindex')
+      ->setName($activity_name)
+      ->setSummary($activity_summary)
+      ->setMessage($activity_message);
+  }
 
+  private function raiseRebuildIdentitiesIssue() {
+    $activity_name = pht('Rebuild Repository Identities');
+    $activity_summary = pht(
+      'The mapping from VCS users to Phabricator users has changed '.
+      'and must be rebuilt.');
+
+    $message = array();
+
+    $message[] = pht(
+      'The way Phabricator attributes VCS activity to Phabricator users '.
+      'has changed. There is a new indirection layer between the strings '.
+      'that appear as VCS authors and committers (such as "John Developer '.
+      '<johnd@bigcorp.com>") and the Phabricator user that gets associated '.
+      'with VCS commits. This is to support situations where users '.
+      'are incorrectly associated with commits by Phabricator making bad '.
+      'guesses about the identity of the corresponding Phabricator user. '.
+      'This also helps with situations where existing repositories are '.
+      'imported without having created accounts for all the committers to '.
+      'that repository. Until you rebuild these repository identities, you '.
+      'are likely to encounter problems with future Phabricator features '.
+      'which will rely on the existence of these identities.');
+
+    $message[] = pht(
+      'You can rebuild repository identities while Phabricator is running.');
+
+    $message[] = pht(
+      'To rebuild identities, run this command:');
+
+    $message[] = phutil_tag(
+      'pre',
+      array(),
+      (string)csprintf(
+        'phabricator/ $ ./bin/repository rebuild-identities --all'));
+
+    $message[] = pht(
+      'You can find more information about this new identity mapping '.
+      'here: %s',
+      phutil_tag(
+        'a',
+        array(
+          'href' => 'https://phurl.io/u/repoIdentities',
+          'target' => '_blank',
+        ),
+        'https://phurl.io/u/repoIdentities'));
+
+    $message[] = pht(
+      'After rebuilding repository identities, run this command to clear '.
+      'this setup warning:');
+
+    $message[] = phutil_tag(
+      'pre',
+      array(),
+      'phabricator/ $ ./bin/config done identities');
+
+    $activity_message = phutil_implode_html("\n\n", $message);
+
+    $this->newIssue('manual.identities')
+      ->setName($activity_name)
+      ->setSummary($activity_summary)
+      ->setMessage($activity_message);
   }
 
 }
diff --git a/src/applications/config/storage/PhabricatorConfigManualActivity.php b/src/applications/config/storage/PhabricatorConfigManualActivity.php
index 44c26f77dd..9952021575 100644
--- a/src/applications/config/storage/PhabricatorConfigManualActivity.php
+++ b/src/applications/config/storage/PhabricatorConfigManualActivity.php
@@ -1,38 +1,40 @@
 <?php
 
 final class PhabricatorConfigManualActivity
   extends PhabricatorConfigEntryDAO {
 
   protected $activityType;
   protected $parameters = array();
 
   const TYPE_REINDEX = 'reindex';
+  const TYPE_IDENTITIES = 'identities';
+
 
   protected function getConfiguration() {
     return array(
       self::CONFIG_TIMESTAMPS => false,
       self::CONFIG_SERIALIZATION => array(
         'parameters' => self::SERIALIZATION_JSON,
       ),
       self::CONFIG_COLUMN_SCHEMA => array(
         'activityType' => 'text64',
       ),
       self::CONFIG_KEY_SCHEMA => array(
         'key_type' => array(
           'columns' => array('activityType'),
           'unique' => true,
         ),
       ),
     ) + parent::getConfiguration();
   }
 
   public function setParameter($key, $value) {
     $this->parameters[$key] = $value;
     return $this;
   }
 
   public function getParameter($key, $default = null) {
     return idx($this->parameters, $key, $default);
   }
 
 }