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
@@ -157,6 +157,7 @@
     'AphrontRedirectResponseTestCase' => 'aphront/response/__tests__/AphrontRedirectResponseTestCase.php',
     'AphrontReloadResponse' => 'aphront/response/AphrontReloadResponse.php',
     'AphrontRequest' => 'aphront/AphrontRequest.php',
+    'AphrontRequestExceptionHandler' => 'aphront/handler/AphrontRequestExceptionHandler.php',
     'AphrontRequestTestCase' => 'aphront/__tests__/AphrontRequestTestCase.php',
     'AphrontResponse' => 'aphront/response/AphrontResponse.php',
     'AphrontResponseProducerInterface' => 'aphront/interface/AphrontResponseProducerInterface.php',
@@ -1484,6 +1485,7 @@
     'PhabricatorActionView' => 'view/layout/PhabricatorActionView.php',
     'PhabricatorActivitySettingsPanel' => 'applications/settings/panel/PhabricatorActivitySettingsPanel.php',
     'PhabricatorAdministratorsPolicyRule' => 'applications/policy/rule/PhabricatorAdministratorsPolicyRule.php',
+    'PhabricatorAjaxRequestExceptionHandler' => 'aphront/handler/PhabricatorAjaxRequestExceptionHandler.php',
     'PhabricatorAlmanacApplication' => 'applications/almanac/application/PhabricatorAlmanacApplication.php',
     'PhabricatorAmazonAuthProvider' => 'applications/auth/provider/PhabricatorAmazonAuthProvider.php',
     'PhabricatorAnchorView' => 'view/layout/PhabricatorAnchorView.php',
@@ -1786,6 +1788,7 @@
     'PhabricatorConduitLogQuery' => 'applications/conduit/query/PhabricatorConduitLogQuery.php',
     'PhabricatorConduitMethodCallLog' => 'applications/conduit/storage/PhabricatorConduitMethodCallLog.php',
     'PhabricatorConduitMethodQuery' => 'applications/conduit/query/PhabricatorConduitMethodQuery.php',
+    'PhabricatorConduitRequestExceptionHandler' => 'aphront/handler/PhabricatorConduitRequestExceptionHandler.php',
     'PhabricatorConduitSearchEngine' => 'applications/conduit/query/PhabricatorConduitSearchEngine.php',
     'PhabricatorConduitTestCase' => '__tests__/PhabricatorConduitTestCase.php',
     'PhabricatorConduitToken' => 'applications/conduit/storage/PhabricatorConduitToken.php',
@@ -1838,6 +1841,7 @@
     'PhabricatorConfigOptionType' => 'applications/config/custom/PhabricatorConfigOptionType.php',
     'PhabricatorConfigPHIDModule' => 'applications/config/module/PhabricatorConfigPHIDModule.php',
     'PhabricatorConfigProxySource' => 'infrastructure/env/PhabricatorConfigProxySource.php',
+    'PhabricatorConfigRequestExceptionHandlerModule' => 'applications/config/module/PhabricatorConfigRequestExceptionHandlerModule.php',
     'PhabricatorConfigResponse' => 'applications/config/response/PhabricatorConfigResponse.php',
     'PhabricatorConfigSchemaQuery' => 'applications/config/schema/PhabricatorConfigSchemaQuery.php',
     'PhabricatorConfigSchemaSpec' => 'applications/config/schema/PhabricatorConfigSchemaSpec.php',
@@ -1993,6 +1997,7 @@
     'PhabricatorDatabaseSetupCheck' => 'applications/config/check/PhabricatorDatabaseSetupCheck.php',
     'PhabricatorDateTimeSettingsPanel' => 'applications/settings/panel/PhabricatorDateTimeSettingsPanel.php',
     'PhabricatorDebugController' => 'applications/system/controller/PhabricatorDebugController.php',
+    'PhabricatorDefaultRequestExceptionHandler' => 'aphront/handler/PhabricatorDefaultRequestExceptionHandler.php',
     'PhabricatorDesktopNotificationsSettingsPanel' => 'applications/settings/panel/PhabricatorDesktopNotificationsSettingsPanel.php',
     'PhabricatorDestructibleInterface' => 'applications/system/interface/PhabricatorDestructibleInterface.php',
     'PhabricatorDestructionEngine' => 'applications/system/engine/PhabricatorDestructionEngine.php',
@@ -2183,6 +2188,7 @@
     'PhabricatorHelpEditorProtocolController' => 'applications/help/controller/PhabricatorHelpEditorProtocolController.php',
     'PhabricatorHelpKeyboardShortcutController' => 'applications/help/controller/PhabricatorHelpKeyboardShortcutController.php',
     'PhabricatorHeraldApplication' => 'applications/herald/application/PhabricatorHeraldApplication.php',
+    'PhabricatorHighSecurityRequestExceptionHandler' => 'aphront/handler/PhabricatorHighSecurityRequestExceptionHandler.php',
     'PhabricatorHomeApplication' => 'applications/home/application/PhabricatorHomeApplication.php',
     'PhabricatorHomeController' => 'applications/home/controller/PhabricatorHomeController.php',
     'PhabricatorHomeMainController' => 'applications/home/controller/PhabricatorHomeMainController.php',
@@ -2580,6 +2586,7 @@
     'PhabricatorPolicyManagementWorkflow' => 'applications/policy/management/PhabricatorPolicyManagementWorkflow.php',
     'PhabricatorPolicyPHIDTypePolicy' => 'applications/policy/phid/PhabricatorPolicyPHIDTypePolicy.php',
     'PhabricatorPolicyQuery' => 'applications/policy/query/PhabricatorPolicyQuery.php',
+    'PhabricatorPolicyRequestExceptionHandler' => 'aphront/handler/PhabricatorPolicyRequestExceptionHandler.php',
     'PhabricatorPolicyRule' => 'applications/policy/rule/PhabricatorPolicyRule.php',
     'PhabricatorPolicyTestCase' => 'applications/policy/__tests__/PhabricatorPolicyTestCase.php',
     'PhabricatorPolicyTestObject' => 'applications/policy/__tests__/PhabricatorPolicyTestObject.php',
@@ -2667,6 +2674,7 @@
     'PhabricatorQueryOrderItem' => 'infrastructure/query/order/PhabricatorQueryOrderItem.php',
     'PhabricatorQueryOrderTestCase' => 'infrastructure/query/order/__tests__/PhabricatorQueryOrderTestCase.php',
     'PhabricatorQueryOrderVector' => 'infrastructure/query/order/PhabricatorQueryOrderVector.php',
+    'PhabricatorRateLimitRequestExceptionHandler' => 'aphront/handler/PhabricatorRateLimitRequestExceptionHandler.php',
     'PhabricatorRecaptchaConfigOptions' => 'applications/config/option/PhabricatorRecaptchaConfigOptions.php',
     'PhabricatorRecipientHasBadgeEdgeType' => 'applications/badges/edge/PhabricatorRecipientHasBadgeEdgeType.php',
     'PhabricatorRedirectController' => 'applications/base/controller/PhabricatorRedirectController.php',
@@ -2758,6 +2766,7 @@
     'PhabricatorRepositoryURITestCase' => 'applications/repository/storage/__tests__/PhabricatorRepositoryURITestCase.php',
     'PhabricatorRepositoryVCSPassword' => 'applications/repository/storage/PhabricatorRepositoryVCSPassword.php',
     'PhabricatorRepositoryVersion' => 'applications/repository/constants/PhabricatorRepositoryVersion.php',
+    'PhabricatorRequestExceptionHandler' => 'aphront/handler/PhabricatorRequestExceptionHandler.php',
     'PhabricatorResourceSite' => 'aphront/site/PhabricatorResourceSite.php',
     'PhabricatorRobotsController' => 'applications/system/controller/PhabricatorRobotsController.php',
     'PhabricatorS3FileStorageEngine' => 'applications/files/engine/PhabricatorS3FileStorageEngine.php',
@@ -3788,6 +3797,7 @@
     'AphrontRedirectResponseTestCase' => 'PhabricatorTestCase',
     'AphrontReloadResponse' => 'AphrontRedirectResponse',
     'AphrontRequest' => 'Phobject',
+    'AphrontRequestExceptionHandler' => 'Phobject',
     'AphrontRequestTestCase' => 'PhabricatorTestCase',
     'AphrontResponse' => 'Phobject',
     'AphrontRoutingMap' => 'Phobject',
@@ -5303,6 +5313,7 @@
     'PhabricatorActionView' => 'AphrontView',
     'PhabricatorActivitySettingsPanel' => 'PhabricatorSettingsPanel',
     'PhabricatorAdministratorsPolicyRule' => 'PhabricatorPolicyRule',
+    'PhabricatorAjaxRequestExceptionHandler' => 'PhabricatorRequestExceptionHandler',
     'PhabricatorAlmanacApplication' => 'PhabricatorApplication',
     'PhabricatorAmazonAuthProvider' => 'PhabricatorOAuth2AuthProvider',
     'PhabricatorAnchorView' => 'AphrontView',
@@ -5667,6 +5678,7 @@
       'PhabricatorPolicyInterface',
     ),
     'PhabricatorConduitMethodQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
+    'PhabricatorConduitRequestExceptionHandler' => 'PhabricatorRequestExceptionHandler',
     'PhabricatorConduitSearchEngine' => 'PhabricatorApplicationSearchEngine',
     'PhabricatorConduitTestCase' => 'PhabricatorTestCase',
     'PhabricatorConduitToken' => array(
@@ -5729,6 +5741,7 @@
     'PhabricatorConfigOptionType' => 'Phobject',
     'PhabricatorConfigPHIDModule' => 'PhabricatorConfigModule',
     'PhabricatorConfigProxySource' => 'PhabricatorConfigSource',
+    'PhabricatorConfigRequestExceptionHandlerModule' => 'PhabricatorConfigModule',
     'PhabricatorConfigResponse' => 'AphrontStandaloneHTMLResponse',
     'PhabricatorConfigSchemaQuery' => 'Phobject',
     'PhabricatorConfigSchemaSpec' => 'Phobject',
@@ -5913,6 +5926,7 @@
     'PhabricatorDatabaseSetupCheck' => 'PhabricatorSetupCheck',
     'PhabricatorDateTimeSettingsPanel' => 'PhabricatorSettingsPanel',
     'PhabricatorDebugController' => 'PhabricatorController',
+    'PhabricatorDefaultRequestExceptionHandler' => 'PhabricatorRequestExceptionHandler',
     'PhabricatorDesktopNotificationsSettingsPanel' => 'PhabricatorSettingsPanel',
     'PhabricatorDestructionEngine' => 'Phobject',
     'PhabricatorDeveloperConfigOptions' => 'PhabricatorApplicationConfigOptions',
@@ -6138,6 +6152,7 @@
     'PhabricatorHelpEditorProtocolController' => 'PhabricatorHelpController',
     'PhabricatorHelpKeyboardShortcutController' => 'PhabricatorHelpController',
     'PhabricatorHeraldApplication' => 'PhabricatorApplication',
+    'PhabricatorHighSecurityRequestExceptionHandler' => 'PhabricatorRequestExceptionHandler',
     'PhabricatorHomeApplication' => 'PhabricatorApplication',
     'PhabricatorHomeController' => 'PhabricatorController',
     'PhabricatorHomeMainController' => 'PhabricatorHomeController',
@@ -6587,6 +6602,7 @@
     'PhabricatorPolicyManagementWorkflow' => 'PhabricatorManagementWorkflow',
     'PhabricatorPolicyPHIDTypePolicy' => 'PhabricatorPHIDType',
     'PhabricatorPolicyQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
+    'PhabricatorPolicyRequestExceptionHandler' => 'PhabricatorRequestExceptionHandler',
     'PhabricatorPolicyRule' => 'Phobject',
     'PhabricatorPolicyTestCase' => 'PhabricatorTestCase',
     'PhabricatorPolicyTestObject' => array(
@@ -6702,6 +6718,7 @@
       'Phobject',
       'Iterator',
     ),
+    'PhabricatorRateLimitRequestExceptionHandler' => 'PhabricatorRequestExceptionHandler',
     'PhabricatorRecaptchaConfigOptions' => 'PhabricatorApplicationConfigOptions',
     'PhabricatorRecipientHasBadgeEdgeType' => 'PhabricatorEdgeType',
     'PhabricatorRedirectController' => 'PhabricatorController',
@@ -6828,6 +6845,7 @@
     'PhabricatorRepositoryURITestCase' => 'PhabricatorTestCase',
     'PhabricatorRepositoryVCSPassword' => 'PhabricatorRepositoryDAO',
     'PhabricatorRepositoryVersion' => 'Phobject',
+    'PhabricatorRequestExceptionHandler' => 'AphrontRequestExceptionHandler',
     'PhabricatorResourceSite' => 'PhabricatorSite',
     'PhabricatorRobotsController' => 'PhabricatorController',
     'PhabricatorS3FileStorageEngine' => 'PhabricatorFileStorageEngine',
diff --git a/src/aphront/configuration/AphrontApplicationConfiguration.php b/src/aphront/configuration/AphrontApplicationConfiguration.php
--- a/src/aphront/configuration/AphrontApplicationConfiguration.php
+++ b/src/aphront/configuration/AphrontApplicationConfiguration.php
@@ -3,6 +3,7 @@
 /**
  * @task routing URI Routing
  * @task response Response Handling
+ * @task exception Exception Handling
  */
 abstract class AphrontApplicationConfiguration extends Phobject {
 
@@ -11,7 +12,6 @@
   private $path;
   private $console;
 
-  abstract public function getApplicationName();
   abstract public function buildRequest();
   abstract public function build404Controller();
   abstract public function buildRedirectController($uri, $external);
@@ -482,7 +482,7 @@
 
 
   /**
-   * Verifies that the erturn value from an
+   * Verifies that the return value from an
    * @{class:AphrontResponseProducerInterface} is of an allowed type.
    *
    * @param AphrontResponseProducerInterface Object which produced
@@ -512,6 +512,36 @@
 
 
   /**
+   * Verifies that the return value from an
+   * @{class:AphrontRequestExceptionHandler} is of an allowed type.
+   *
+   * @param AphrontRequestExceptionHandler Object which produced this
+   *  response.
+   * @param wild Supposedly valid response.
+   * @return void
+   * @task response
+   */
+  private function validateErrorHandlerResponse(
+    AphrontRequestExceptionHandler $handler,
+    $response) {
+
+    if ($this->isValidResponseObject($response)) {
+      return;
+    }
+
+    throw new Exception(
+      pht(
+        'Exception handler "%s" returned an invalid response from call to '.
+        '"%s". This method must return an object of class "%s", or an object '.
+        'which implements the "%s" interface.',
+        get_class($handler),
+        'handleRequestException()',
+        'AphrontResponse',
+        'AphrontResponseProducerInterface'));
+  }
+
+
+  /**
    * Resolves a response object into an @{class:AphrontResponse}.
    *
    * Controllers are permitted to return actual responses of class
@@ -572,4 +602,34 @@
   }
 
 
+/* -(  Error Handling  )----------------------------------------------------- */
+
+
+  /**
+   * Convert an exception which has escaped the controller into a response.
+   *
+   * This method delegates exception handling to available subclasses of
+   * @{class:AphrontRequestExceptionHandler}.
+   *
+   * @param Exception Exception which needs to be handled.
+   * @return wild Response or response producer, or null if no available
+   *   handler can produce a response.
+   * @task exception
+   */
+  private function handleException(Exception $ex) {
+    $handlers = AphrontRequestExceptionHandler::getAllHandlers();
+
+    $request = $this->getRequest();
+    foreach ($handlers as $handler) {
+      if ($handler->canHandleRequestException($request, $ex)) {
+        $response = $handler->handleRequestException($request, $ex);
+        $this->validateErrorHandlerResponse($handler, $response);
+        return $response;
+      }
+    }
+
+    throw $ex;
+  }
+
+
 }
diff --git a/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php b/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php
--- a/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php
+++ b/src/aphront/configuration/AphrontDefaultApplicationConfiguration.php
@@ -8,12 +8,6 @@
 class AphrontDefaultApplicationConfiguration
   extends AphrontApplicationConfiguration {
 
-  public function __construct() {}
-
-  public function getApplicationName() {
-    return 'aphront-default';
-  }
-
   /**
    * @phutil-external-symbol class PhabricatorStartup
    */
@@ -50,213 +44,6 @@
     return $request;
   }
 
-  public function handleException(Exception $ex) {
-    $request = $this->getRequest();
-
-    // For Conduit requests, return a Conduit response.
-    if ($request->isConduit()) {
-      $response = new ConduitAPIResponse();
-      $response->setErrorCode(get_class($ex));
-      $response->setErrorInfo($ex->getMessage());
-
-      return id(new AphrontJSONResponse())
-        ->setAddJSONShield(false)
-        ->setContent($response->toDictionary());
-    }
-
-    // For non-workflow requests, return a Ajax response.
-    if ($request->isAjax() && !$request->isWorkflow()) {
-      // Log these; they don't get shown on the client and can be difficult
-      // to debug.
-      phlog($ex);
-
-      $response = new AphrontAjaxResponse();
-      $response->setError(
-        array(
-          'code' => get_class($ex),
-          'info' => $ex->getMessage(),
-        ));
-      return $response;
-    }
-
-    $user = $request->getUser();
-    if (!$user) {
-      // If we hit an exception very early, we won't have a user.
-      $user = new PhabricatorUser();
-    }
-
-    if ($ex instanceof PhabricatorSystemActionRateLimitException) {
-      $dialog = id(new AphrontDialogView())
-        ->setTitle(pht('Slow Down!'))
-        ->setUser($user)
-        ->setErrors(array(pht('You are being rate limited.')))
-        ->appendParagraph($ex->getMessage())
-        ->appendParagraph($ex->getRateExplanation())
-        ->addCancelButton('/', pht('Okaaaaaaaaaaaaaay...'));
-
-      $response = new AphrontDialogResponse();
-      $response->setDialog($dialog);
-      return $response;
-    }
-
-    if ($ex instanceof PhabricatorAuthHighSecurityRequiredException) {
-
-      $form = id(new PhabricatorAuthSessionEngine())->renderHighSecurityForm(
-        $ex->getFactors(),
-        $ex->getFactorValidationResults(),
-        $user,
-        $request);
-
-      $dialog = id(new AphrontDialogView())
-        ->setUser($user)
-        ->setTitle(pht('Entering High Security'))
-        ->setShortTitle(pht('Security Checkpoint'))
-        ->setWidth(AphrontDialogView::WIDTH_FORM)
-        ->addHiddenInput(AphrontRequest::TYPE_HISEC, true)
-        ->setErrors(
-          array(
-            pht(
-              'You are taking an action which requires you to enter '.
-              'high security.'),
-          ))
-        ->appendParagraph(
-          pht(
-            'High security mode helps protect your account from security '.
-            'threats, like session theft or someone messing with your stuff '.
-            'while you\'re grabbing a coffee. To enter high security mode, '.
-            'confirm your credentials.'))
-        ->appendChild($form->buildLayoutView())
-        ->appendParagraph(
-          pht(
-            'Your account will remain in high security mode for a short '.
-            'period of time. When you are finished taking sensitive '.
-            'actions, you should leave high security.'))
-        ->setSubmitURI($request->getPath())
-        ->addCancelButton($ex->getCancelURI())
-        ->addSubmitButton(pht('Enter High Security'));
-
-      $request_parameters = $request->getPassthroughRequestParameters(
-        $respect_quicksand = true);
-      foreach ($request_parameters as $key => $value) {
-        $dialog->addHiddenInput($key, $value);
-      }
-
-      $response = new AphrontDialogResponse();
-      $response->setDialog($dialog);
-      return $response;
-    }
-
-    if ($ex instanceof PhabricatorPolicyException) {
-      if (!$user->isLoggedIn()) {
-        // If the user isn't logged in, just give them a login form. This is
-        // probably a generally more useful response than a policy dialog that
-        // they have to click through to get a login form.
-        //
-        // Possibly we should add a header here like "you need to login to see
-        // the thing you are trying to look at".
-        $login_controller = new PhabricatorAuthStartController();
-        $login_controller->setRequest($request);
-
-        $auth_app_class = 'PhabricatorAuthApplication';
-        $auth_app = PhabricatorApplication::getByClass($auth_app_class);
-        $login_controller->setCurrentApplication($auth_app);
-
-        return $login_controller->handleRequest($request);
-      }
-
-      $content = array(
-        phutil_tag(
-          'div',
-          array(
-            'class' => 'aphront-policy-rejection',
-          ),
-          $ex->getRejection()),
-      );
-
-      $list = null;
-      if ($ex->getCapabilityName()) {
-        $list = $ex->getMoreInfo();
-        foreach ($list as $key => $item) {
-          $list[$key] = $item;
-        }
-
-        $content[] = phutil_tag(
-          'div',
-          array(
-            'class' => 'aphront-capability-details',
-          ),
-          pht('Users with the "%s" capability:', $ex->getCapabilityName()));
-
-      }
-
-      $dialog = id(new AphrontDialogView())
-        ->setTitle($ex->getTitle())
-        ->setClass('aphront-access-dialog')
-        ->setUser($user)
-        ->appendChild($content);
-
-      if ($list) {
-        $dialog->appendList($list);
-      }
-
-      if ($this->getRequest()->isAjax()) {
-        $dialog->addCancelButton('/', pht('Close'));
-      } else {
-        $dialog->addCancelButton('/', pht('OK'));
-      }
-
-      $response = new AphrontDialogResponse();
-      $response->setDialog($dialog);
-      return $response;
-    }
-
-    // Always log the unhandled exception.
-    phlog($ex);
-
-    $class    = get_class($ex);
-    $message  = $ex->getMessage();
-
-    if ($ex instanceof AphrontSchemaQueryException) {
-      $message .= "\n\n".pht(
-        "NOTE: This usually indicates that the MySQL schema has not been ".
-        "properly upgraded. Run '%s' to ensure your schema is up to date.",
-        'bin/storage upgrade');
-    }
-
-    if (PhabricatorEnv::getEnvConfig('phabricator.developer-mode')) {
-      $trace = id(new AphrontStackTraceView())
-        ->setUser($user)
-        ->setTrace($ex->getTrace());
-    } else {
-      $trace = null;
-    }
-
-    $content = phutil_tag(
-      'div',
-      array('class' => 'aphront-unhandled-exception'),
-      array(
-        phutil_tag('div', array('class' => 'exception-message'), $message),
-        $trace,
-      ));
-
-    $dialog = new AphrontDialogView();
-    $dialog
-      ->setTitle(pht('Unhandled Exception ("%s")', $class))
-      ->setClass('aphront-exception-dialog')
-      ->setUser($user)
-      ->appendChild($content);
-
-    if ($this->getRequest()->isAjax()) {
-      $dialog->addCancelButton('/', pht('Close'));
-    }
-
-    $response = new AphrontDialogResponse();
-    $response->setDialog($dialog);
-    $response->setHTTPResponseCode(500);
-
-    return $response;
-  }
-
   public function build404Controller() {
     return array(new Phabricator404Controller(), array());
   }
diff --git a/src/aphront/handler/AphrontRequestExceptionHandler.php b/src/aphront/handler/AphrontRequestExceptionHandler.php
new file mode 100644
--- /dev/null
+++ b/src/aphront/handler/AphrontRequestExceptionHandler.php
@@ -0,0 +1,36 @@
+<?php
+
+/**
+ * React to an unhandled exception escaping request handling in a controller
+ * and convert it into a response.
+ *
+ * These handlers are generally used to render error pages, but they may
+ * also perform more specialized handling in situations where an error page
+ * is not appropriate.
+ */
+abstract class AphrontRequestExceptionHandler extends Phobject {
+
+  abstract public function getRequestExceptionHandlerPriority();
+
+  public function shouldLogException(
+    AphrontRequest $request,
+    Exception $ex) {
+    return null;
+  }
+
+  abstract public function canHandleRequestException(
+    AphrontRequest $request,
+    Exception $ex);
+
+  abstract public function handleRequestException(
+    AphrontRequest $request,
+    Exception $ex);
+
+  final public static function getAllHandlers() {
+    return id(new PhutilClassMapQuery())
+      ->setAncestorClass(__CLASS__)
+      ->setSortMethod('getRequestExceptionHandlerPriority')
+      ->execute();
+  }
+
+}
diff --git a/src/aphront/handler/PhabricatorAjaxRequestExceptionHandler.php b/src/aphront/handler/PhabricatorAjaxRequestExceptionHandler.php
new file mode 100644
--- /dev/null
+++ b/src/aphront/handler/PhabricatorAjaxRequestExceptionHandler.php
@@ -0,0 +1,38 @@
+<?php
+
+final class PhabricatorAjaxRequestExceptionHandler
+  extends PhabricatorRequestExceptionHandler {
+
+  public function getRequestExceptionHandlerPriority() {
+    return 110000;
+  }
+
+  public function getRequestExceptionHandlerDescription() {
+    return pht('Responds to requests made by AJAX clients.');
+  }
+
+  public function canHandleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+    // For non-workflow requests, return a Ajax response.
+    return ($request->isAjax() && !$request->isWorkflow());
+  }
+
+  public function handleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    // Log these; they don't get shown on the client and can be difficult
+    // to debug.
+    phlog($ex);
+
+    $response = new AphrontAjaxResponse();
+    $response->setError(
+      array(
+        'code' => get_class($ex),
+        'info' => $ex->getMessage(),
+      ));
+    return $response;
+  }
+
+}
diff --git a/src/aphront/handler/PhabricatorConduitRequestExceptionHandler.php b/src/aphront/handler/PhabricatorConduitRequestExceptionHandler.php
new file mode 100644
--- /dev/null
+++ b/src/aphront/handler/PhabricatorConduitRequestExceptionHandler.php
@@ -0,0 +1,33 @@
+<?php
+
+final class PhabricatorConduitRequestExceptionHandler
+  extends PhabricatorRequestExceptionHandler {
+
+  public function getRequestExceptionHandlerPriority() {
+    return 100000;
+  }
+
+  public function getRequestExceptionHandlerDescription() {
+    return pht('Responds to requests made by Conduit clients.');
+  }
+
+  public function canHandleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+    return $request->isConduit();
+  }
+
+  public function handleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    $response = id(new ConduitAPIResponse())
+      ->setErrorCode(get_class($ex))
+      ->setErrorInfo($ex->getMessage());
+
+    return id(new AphrontJSONResponse())
+      ->setAddJSONShield(false)
+      ->setContent($response->toDictionary());
+  }
+
+}
diff --git a/src/aphront/handler/PhabricatorDefaultRequestExceptionHandler.php b/src/aphront/handler/PhabricatorDefaultRequestExceptionHandler.php
new file mode 100644
--- /dev/null
+++ b/src/aphront/handler/PhabricatorDefaultRequestExceptionHandler.php
@@ -0,0 +1,76 @@
+<?php
+
+final class PhabricatorDefaultRequestExceptionHandler
+  extends PhabricatorRequestExceptionHandler {
+
+  public function getRequestExceptionHandlerPriority() {
+    return 900000;
+  }
+
+  public function getRequestExceptionHandlerDescription() {
+    return pht('Handles all other exceptions.');
+  }
+
+  public function canHandleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    if (!$this->isPhabricatorSite($request)) {
+      return false;
+    }
+
+    return true;
+  }
+
+  public function handleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    $viewer = $this->getViewer($request);
+
+    // Always log the unhandled exception.
+    phlog($ex);
+
+    $class = get_class($ex);
+    $message = $ex->getMessage();
+
+    if ($ex instanceof AphrontSchemaQueryException) {
+      $message .= "\n\n".pht(
+        "NOTE: This usually indicates that the MySQL schema has not been ".
+        "properly upgraded. Run '%s' to ensure your schema is up to date.",
+        'bin/storage upgrade');
+    }
+
+    if (PhabricatorEnv::getEnvConfig('phabricator.developer-mode')) {
+      $trace = id(new AphrontStackTraceView())
+        ->setUser($viewer)
+        ->setTrace($ex->getTrace());
+    } else {
+      $trace = null;
+    }
+
+    $content = phutil_tag(
+      'div',
+      array('class' => 'aphront-unhandled-exception'),
+      array(
+        phutil_tag('div', array('class' => 'exception-message'), $message),
+        $trace,
+      ));
+
+    $dialog = new AphrontDialogView();
+    $dialog
+      ->setTitle(pht('Unhandled Exception ("%s")', $class))
+      ->setClass('aphront-exception-dialog')
+      ->setUser($viewer)
+      ->appendChild($content);
+
+    if ($request->isAjax()) {
+      $dialog->addCancelButton('/', pht('Close'));
+    }
+
+    return id(new AphrontDialogResponse())
+      ->setDialog($dialog)
+      ->setHTTPResponseCode(500);
+  }
+
+}
diff --git a/src/aphront/handler/PhabricatorHighSecurityRequestExceptionHandler.php b/src/aphront/handler/PhabricatorHighSecurityRequestExceptionHandler.php
new file mode 100644
--- /dev/null
+++ b/src/aphront/handler/PhabricatorHighSecurityRequestExceptionHandler.php
@@ -0,0 +1,76 @@
+<?php
+
+final class PhabricatorHighSecurityRequestExceptionHandler
+  extends PhabricatorRequestExceptionHandler {
+
+  public function getRequestExceptionHandlerPriority() {
+    return 310000;
+  }
+
+  public function getRequestExceptionHandlerDescription() {
+    return pht(
+      'Handles high security exceptions which occur when a user needs '.
+      'to present MFA credentials to take an action.');
+  }
+
+  public function canHandleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    if (!$this->isPhabricatorSite($request)) {
+      return false;
+    }
+
+    return ($ex instanceof PhabricatorAuthHighSecurityRequiredException);
+  }
+
+  public function handleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    $viewer = $this->getViewer($request);
+
+    $form = id(new PhabricatorAuthSessionEngine())->renderHighSecurityForm(
+      $ex->getFactors(),
+      $ex->getFactorValidationResults(),
+      $viewer,
+      $request);
+
+    $dialog = id(new AphrontDialogView())
+      ->setUser($viewer)
+      ->setTitle(pht('Entering High Security'))
+      ->setShortTitle(pht('Security Checkpoint'))
+      ->setWidth(AphrontDialogView::WIDTH_FORM)
+      ->addHiddenInput(AphrontRequest::TYPE_HISEC, true)
+      ->setErrors(
+        array(
+          pht(
+            'You are taking an action which requires you to enter '.
+            'high security.'),
+        ))
+      ->appendParagraph(
+        pht(
+          'High security mode helps protect your account from security '.
+          'threats, like session theft or someone messing with your stuff '.
+          'while you\'re grabbing a coffee. To enter high security mode, '.
+          'confirm your credentials.'))
+      ->appendChild($form->buildLayoutView())
+      ->appendParagraph(
+        pht(
+          'Your account will remain in high security mode for a short '.
+          'period of time. When you are finished taking sensitive '.
+          'actions, you should leave high security.'))
+      ->setSubmitURI($request->getPath())
+      ->addCancelButton($ex->getCancelURI())
+      ->addSubmitButton(pht('Enter High Security'));
+
+    $request_parameters = $request->getPassthroughRequestParameters(
+      $respect_quicksand = true);
+    foreach ($request_parameters as $key => $value) {
+      $dialog->addHiddenInput($key, $value);
+    }
+
+    return $dialog;
+  }
+
+}
diff --git a/src/aphront/handler/PhabricatorPolicyRequestExceptionHandler.php b/src/aphront/handler/PhabricatorPolicyRequestExceptionHandler.php
new file mode 100644
--- /dev/null
+++ b/src/aphront/handler/PhabricatorPolicyRequestExceptionHandler.php
@@ -0,0 +1,93 @@
+<?php
+
+final class PhabricatorPolicyRequestExceptionHandler
+  extends PhabricatorRequestExceptionHandler {
+
+  public function getRequestExceptionHandlerPriority() {
+    return 320000;
+  }
+
+  public function getRequestExceptionHandlerDescription() {
+    return pht(
+      'Handles policy exceptions which occur when a user tries to '.
+      'do something they do not have permission to do.');
+  }
+
+  public function canHandleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    if (!$this->isPhabricatorSite($request)) {
+      return false;
+    }
+
+    return ($ex instanceof PhabricatorPolicyException);
+  }
+
+  public function handleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    $viewer = $this->getViewer($request);
+
+    if (!$viewer->isLoggedIn()) {
+      // If the user isn't logged in, just give them a login form. This is
+      // probably a generally more useful response than a policy dialog that
+      // they have to click through to get a login form.
+      //
+      // Possibly we should add a header here like "you need to login to see
+      // the thing you are trying to look at".
+      $auth_app_class = 'PhabricatorAuthApplication';
+      $auth_app = PhabricatorApplication::getByClass($auth_app_class);
+
+      return id(new PhabricatorAuthStartController())
+        ->setRequest($request)
+        ->setCurrentApplication($auth_app)
+        ->handleRequest($request);
+    }
+
+    $content = array(
+      phutil_tag(
+        'div',
+        array(
+          'class' => 'aphront-policy-rejection',
+        ),
+        $ex->getRejection()),
+    );
+
+    $list = null;
+    if ($ex->getCapabilityName()) {
+      $list = $ex->getMoreInfo();
+      foreach ($list as $key => $item) {
+        $list[$key] = $item;
+      }
+
+      $content[] = phutil_tag(
+        'div',
+        array(
+          'class' => 'aphront-capability-details',
+        ),
+        pht('Users with the "%s" capability:', $ex->getCapabilityName()));
+
+    }
+
+    $dialog = id(new AphrontDialogView())
+      ->setTitle($ex->getTitle())
+      ->setClass('aphront-access-dialog')
+      ->setUser($viewer)
+      ->appendChild($content);
+
+    if ($list) {
+      $dialog->appendList($list);
+    }
+
+    if ($request->isAjax()) {
+      $dialog->addCancelButton('/', pht('Close'));
+    } else {
+      $dialog->addCancelButton('/', pht('OK'));
+    }
+
+    return $dialog;
+  }
+
+}
diff --git a/src/aphront/handler/PhabricatorRateLimitRequestExceptionHandler.php b/src/aphront/handler/PhabricatorRateLimitRequestExceptionHandler.php
new file mode 100644
--- /dev/null
+++ b/src/aphront/handler/PhabricatorRateLimitRequestExceptionHandler.php
@@ -0,0 +1,42 @@
+<?php
+
+final class PhabricatorRateLimitRequestExceptionHandler
+  extends PhabricatorRequestExceptionHandler {
+
+  public function getRequestExceptionHandlerPriority() {
+    return 300000;
+  }
+
+  public function getRequestExceptionHandlerDescription() {
+    return pht(
+      'Handles action rate limiting exceptions which occur when a user '.
+      'does something too frequently.');
+  }
+
+  public function canHandleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    if (!$this->isPhabricatorSite($request)) {
+      return false;
+    }
+
+    return ($ex instanceof PhabricatorSystemActionRateLimitException);
+  }
+
+  public function handleRequestException(
+    AphrontRequest $request,
+    Exception $ex) {
+
+    $viewer = $this->getViewer($request);
+
+    return id(new AphrontDialogView())
+      ->setTitle(pht('Slow Down!'))
+      ->setUser($viewer)
+      ->setErrors(array(pht('You are being rate limited.')))
+      ->appendParagraph($ex->getMessage())
+      ->appendParagraph($ex->getRateExplanation())
+      ->addCancelButton('/', pht('Okaaaaaaaaaaaaaay...'));
+  }
+
+}
diff --git a/src/aphront/handler/PhabricatorRequestExceptionHandler.php b/src/aphront/handler/PhabricatorRequestExceptionHandler.php
new file mode 100644
--- /dev/null
+++ b/src/aphront/handler/PhabricatorRequestExceptionHandler.php
@@ -0,0 +1,26 @@
+<?php
+
+abstract class PhabricatorRequestExceptionHandler
+  extends AphrontRequestExceptionHandler {
+
+  protected function isPhabricatorSite(AphrontRequest $request) {
+    $site = $request->getSite();
+    if (!$site) {
+      return false;
+    }
+
+    return ($site instanceof PhabricatorSite);
+  }
+
+  protected function getViewer(AphrontRequest $request) {
+    $viewer = $request->getUser();
+
+    if ($viewer) {
+      return $viewer;
+    }
+
+    // If we hit an exception very early, we won't have a user yet.
+    return new PhabricatorUser();
+  }
+
+}
diff --git a/src/applications/config/module/PhabricatorConfigEdgeModule.php b/src/applications/config/module/PhabricatorConfigEdgeModule.php
--- a/src/applications/config/module/PhabricatorConfigEdgeModule.php
+++ b/src/applications/config/module/PhabricatorConfigEdgeModule.php
@@ -41,7 +41,7 @@
 
     return id(new PHUIObjectBoxView())
       ->setHeaderText(pht('Edge Types'))
-      ->appendChild($table);
+      ->setTable($table);
   }
 
 }
diff --git a/src/applications/config/module/PhabricatorConfigPHIDModule.php b/src/applications/config/module/PhabricatorConfigPHIDModule.php
--- a/src/applications/config/module/PhabricatorConfigPHIDModule.php
+++ b/src/applications/config/module/PhabricatorConfigPHIDModule.php
@@ -41,7 +41,7 @@
 
     return id(new PHUIObjectBoxView())
       ->setHeaderText(pht('PHID Types'))
-      ->appendChild($table);
+      ->setTable($table);
   }
 
 }
diff --git a/src/applications/config/module/PhabricatorConfigSiteModule.php b/src/applications/config/module/PhabricatorConfigRequestExceptionHandlerModule.php
copy from src/applications/config/module/PhabricatorConfigSiteModule.php
copy to src/applications/config/module/PhabricatorConfigRequestExceptionHandlerModule.php
--- a/src/applications/config/module/PhabricatorConfigSiteModule.php
+++ b/src/applications/config/module/PhabricatorConfigRequestExceptionHandlerModule.php
@@ -1,26 +1,27 @@
 <?php
 
-final class PhabricatorConfigSiteModule extends PhabricatorConfigModule {
+final class PhabricatorConfigRequestExceptionHandlerModule
+  extends PhabricatorConfigModule {
 
   public function getModuleKey() {
-    return 'site';
+    return 'exception-handler';
   }
 
   public function getModuleName() {
-    return pht('Sites');
+    return pht('Exception Handlers');
   }
 
   public function renderModuleStatus(AphrontRequest $request) {
     $viewer = $request->getViewer();
 
-    $sites = AphrontSite::getAllSites();
+    $handlers = AphrontRequestExceptionHandler::getAllHandlers();
 
     $rows = array();
-    foreach ($sites as $key => $site) {
+    foreach ($handlers as $key => $handler) {
       $rows[] = array(
-        $site->getPriority(),
+        $handler->getRequestExceptionHandlerPriority(),
         $key,
-        $site->getDescription(),
+        $handler->getRequestExceptionHandlerDescription(),
       );
     }
 
@@ -39,8 +40,8 @@
         ));
 
     return id(new PHUIObjectBoxView())
-      ->setHeaderText(pht('Sites'))
-      ->appendChild($table);
+      ->setHeaderText(pht('Exception Handlers'))
+      ->setTable($table);
   }
 
 }
diff --git a/src/applications/config/module/PhabricatorConfigSiteModule.php b/src/applications/config/module/PhabricatorConfigSiteModule.php
--- a/src/applications/config/module/PhabricatorConfigSiteModule.php
+++ b/src/applications/config/module/PhabricatorConfigSiteModule.php
@@ -40,7 +40,7 @@
 
     return id(new PHUIObjectBoxView())
       ->setHeaderText(pht('Sites'))
-      ->appendChild($table);
+      ->setTable($table);
   }
 
 }