Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -355,6 +355,7 @@
     'DifferentialCommitsFieldSpecification' => 'applications/differential/field/specification/DifferentialCommitsFieldSpecification.php',
     'DifferentialConflictsFieldSpecification' => 'applications/differential/field/specification/DifferentialConflictsFieldSpecification.php',
     'DifferentialController' => 'applications/differential/controller/DifferentialController.php',
+    'DifferentialCoreCustomField' => 'applications/differential/customfield/DifferentialCoreCustomField.php',
     'DifferentialCustomField' => 'applications/differential/customfield/DifferentialCustomField.php',
     'DifferentialCustomFieldDependsOnParser' => 'applications/differential/field/parser/DifferentialCustomFieldDependsOnParser.php',
     'DifferentialCustomFieldDependsOnParserTestCase' => 'applications/differential/field/parser/__tests__/DifferentialCustomFieldDependsOnParserTestCase.php',
@@ -461,6 +462,7 @@
     'DifferentialRevisionViewController' => 'applications/differential/controller/DifferentialRevisionViewController.php',
     'DifferentialSearchIndexer' => 'applications/differential/search/DifferentialSearchIndexer.php',
     'DifferentialSubscribeController' => 'applications/differential/controller/DifferentialSubscribeController.php',
+    'DifferentialSummaryField' => 'applications/differential/customfield/DifferentialSummaryField.php',
     'DifferentialSummaryFieldSpecification' => 'applications/differential/field/specification/DifferentialSummaryFieldSpecification.php',
     'DifferentialTasksAttacher' => 'applications/differential/DifferentialTasksAttacher.php',
     'DifferentialTestPlanFieldSpecification' => 'applications/differential/field/specification/DifferentialTestPlanFieldSpecification.php',
@@ -2891,6 +2893,7 @@
     'DifferentialCommitsFieldSpecification' => 'DifferentialFieldSpecification',
     'DifferentialConflictsFieldSpecification' => 'DifferentialFieldSpecification',
     'DifferentialController' => 'PhabricatorController',
+    'DifferentialCoreCustomField' => 'DifferentialCustomField',
     'DifferentialCustomField' => 'PhabricatorCustomField',
     'DifferentialCustomFieldDependsOnParser' => 'PhabricatorCustomFieldMonogramParser',
     'DifferentialCustomFieldDependsOnParserTestCase' => 'PhabricatorTestCase',
@@ -3003,9 +3006,10 @@
     'DifferentialRevisionViewController' => 'DifferentialController',
     'DifferentialSearchIndexer' => 'PhabricatorSearchDocumentIndexer',
     'DifferentialSubscribeController' => 'DifferentialController',
+    'DifferentialSummaryField' => 'DifferentialCoreCustomField',
     'DifferentialSummaryFieldSpecification' => 'DifferentialFreeformFieldSpecification',
     'DifferentialTestPlanFieldSpecification' => 'DifferentialFieldSpecification',
-    'DifferentialTitleField' => 'DifferentialCustomField',
+    'DifferentialTitleField' => 'DifferentialCoreCustomField',
     'DifferentialTitleFieldSpecification' => 'DifferentialFreeformFieldSpecification',
     'DifferentialTransaction' => 'PhabricatorApplicationTransaction',
     'DifferentialTransactionComment' => 'PhabricatorApplicationTransactionComment',
Index: src/applications/differential/customfield/DifferentialCoreCustomField.php
===================================================================
--- /dev/null
+++ src/applications/differential/customfield/DifferentialCoreCustomField.php
@@ -0,0 +1,75 @@
+<?php
+
+/**
+ * Base class for Differential fields with storage on the revision object
+ * itself. This mostly wraps reading/writing field values to and from the
+ * object.
+ */
+abstract class DifferentialCoreCustomField
+  extends DifferentialCustomField {
+
+  private $value;
+  private $fieldError;
+
+  abstract protected function readValueFromRevision(
+    DifferentialRevision $revision);
+
+  abstract protected function writeValueToRevision(
+    DifferentialRevision $revision,
+    $value);
+
+  public function isCoreFieldRequired() {
+    return false;
+  }
+
+  public function canDisableField() {
+    return false;
+  }
+
+  public function shouldAppearInApplicationTransactions() {
+    return true;
+  }
+
+  public function shouldAppearInEditView() {
+    return true;
+  }
+
+  protected function didSetObject(PhabricatorCustomFieldInterface $object) {
+    if ($this->isCoreFieldRequired()) {
+      $this->setFieldError(true);
+    }
+    $this->setValue($this->readValueFromRevision($object));
+  }
+
+  public function getOldValueForApplicationTransactions() {
+    return $this->readValueFromRevision($this->getObject());
+  }
+
+  public function getNewValueForApplicationTransactions() {
+    return $this->getValue();
+  }
+
+  public function applyApplicationTransactionInternalEffects(
+    PhabricatorApplicationTransaction $xaction) {
+    $this->writeValueToRevision($this->getObject(), $xaction->getNewValue());
+  }
+
+  public function setFieldError($field_error) {
+    $this->fieldError = $field_error;
+    return $this;
+  }
+
+  public function getFieldError() {
+    return $this->fieldError;
+  }
+
+  public function setValue($value) {
+    $this->value = $value;
+    return $this;
+  }
+
+  public function getValue() {
+    return $this->value;
+  }
+
+}
Index: src/applications/differential/customfield/DifferentialSummaryField.php
===================================================================
--- /dev/null
+++ src/applications/differential/customfield/DifferentialSummaryField.php
@@ -0,0 +1,69 @@
+<?php
+
+final class DifferentialSummaryField
+  extends DifferentialCoreCustomField {
+
+  public function getFieldKey() {
+    return 'differential:summary';
+  }
+
+  public function getFieldName() {
+    return pht('Summary');
+  }
+
+  public function getFieldDescription() {
+    return pht('Stores a summary of the revision.');
+  }
+
+  protected function readValueFromRevision(
+    DifferentialRevision $revision) {
+    return $revision->getSummary();
+  }
+
+  protected function writeValueToRevision(
+    DifferentialRevision $revision,
+    $value) {
+    $revision->setSummary($value);
+  }
+
+  public function readValueFromRequest(AphrontRequest $request) {
+    $this->setValue($request->getStr($this->getFieldKey()));
+  }
+
+  public function renderEditControl() {
+    return id(new PhabricatorRemarkupControl())
+      ->setName($this->getFieldKey())
+      ->setValue($this->getValue())
+      ->setError($this->getFieldError())
+      ->setLabel($this->getFieldName());
+  }
+
+  public function getApplicationTransactionTitle(
+    PhabricatorApplicationTransaction $xaction) {
+    $author_phid = $xaction->getAuthorPHID();
+    $old = $xaction->getOldValue();
+    $new = $xaction->getNewValue();
+
+    return pht(
+      '%s updated the summary for this revision.',
+      $xaction->renderHandleLink($author_phid));
+  }
+
+  public function getApplicationTransactionTitleForFeed(
+    PhabricatorApplicationTransaction $xaction,
+    PhabricatorFeedStory $story) {
+
+    $object_phid = $xaction->getObjectPHID();
+    $author_phid = $xaction->getAuthorPHID();
+    $old = $xaction->getOldValue();
+    $new = $xaction->getNewValue();
+
+    return pht(
+      '%s updated the summary for %s.',
+      $xaction->renderHandleLink($author_phid),
+      $xaction->renderHandleLink($object_phid));
+  }
+
+  // TODO: Support hasChangeDetails() in CustomFields.
+
+}
Index: src/applications/differential/customfield/DifferentialTitleField.php
===================================================================
--- src/applications/differential/customfield/DifferentialTitleField.php
+++ src/applications/differential/customfield/DifferentialTitleField.php
@@ -1,19 +1,7 @@
 <?php
 
 final class DifferentialTitleField
-  extends DifferentialCustomField {
-
-  private $value;
-  private $fieldError = true;
-
-  public function setFieldError($field_error) {
-    $this->fieldError = $field_error;
-    return $this;
-  }
-
-  public function getFieldError() {
-    return $this->fieldError;
-  }
+  extends DifferentialCoreCustomField {
 
   public function getFieldKey() {
     return 'differential:title';
@@ -27,28 +15,15 @@
     return pht('Stores the revision title.');
   }
 
-  public function canDisableField() {
-    return false;
-  }
-
-  public function shouldAppearInApplicationTransactions() {
-    return true;
-  }
-
-  public function shouldAppearInEditView() {
-    return true;
+  protected function readValueFromRevision(
+    DifferentialRevision $revision) {
+    return $revision->getTitle();
   }
 
-  protected function didSetObject(PhabricatorCustomFieldInterface $object) {
-    $this->value = $object->getTitle();
-  }
-
-  public function getOldValueForApplicationTransactions() {
-    return $this->getObject()->getTitle();
-  }
-
-  public function getNewValueForApplicationTransactions() {
-    return $this->value;
+  protected function writeValueToRevision(
+    DifferentialRevision $revision,
+    $value) {
+    $revision->setTitle($value);
   }
 
   public function validateApplicationTransactions(
@@ -77,20 +52,15 @@
     }
   }
 
-  public function applyApplicationTransactionInternalEffects(
-    PhabricatorApplicationTransaction $xaction) {
-    $this->getObject()->setTitle($xaction->getNewValue());
-  }
-
   public function readValueFromRequest(AphrontRequest $request) {
-    $this->value = $request->getStr($this->getFieldKey());
+    $this->setValue($request->getStr($this->getFieldKey()));
   }
 
   public function renderEditControl() {
     return id(new AphrontFormTextAreaControl())
       ->setHeight(AphrontFormTextAreaControl::HEIGHT_VERY_SHORT)
       ->setName($this->getFieldKey())
-      ->setValue($this->value)
+      ->setValue($this->getValue())
       ->setError($this->getFieldError())
       ->setLabel($this->getFieldName());
   }
@@ -138,5 +108,4 @@
     }
   }
 
-
 }
Index: src/applications/differential/storage/DifferentialRevision.php
===================================================================
--- src/applications/differential/storage/DifferentialRevision.php
+++ src/applications/differential/storage/DifferentialRevision.php
@@ -465,10 +465,13 @@
 
 
   public function getCustomFieldSpecificationForRole($role) {
-    return array_fill_keys(
-      array(
+    $fields = array(
+      new DifferentialTitleField(),
+      new DifferentialSummaryField(),
+    );
 
-      ),
+    return array_fill_keys(
+      mpull($fields, 'getFieldKey'),
       array('disabled' => false));
   }