diff --git a/src/Controller/MigrateController.php b/src/Controller/MigrateController.php
index b0d13d2..296a245 100644
--- a/src/Controller/MigrateController.php
+++ b/src/Controller/MigrateController.php
@@ -9,12 +9,15 @@ namespace Drupal\migrate_upgrade\Controller;
 use Drupal\Core\Controller\ControllerBase;
 
 /**
- * Controller methods for the migration.
+ * Provides controller methods for the migration.
  */
 class MigrateController extends ControllerBase {
 
   /**
    * Sets a log filter and redirects to the log.
+   *
+   * @return \Symfony\Component\HttpFoundation\RedirectResponse
+   *   A redirect response object that may be returned by the controller.
    */
   public function showLog() {
     $_SESSION['dblog_overview_filter'] = [];
diff --git a/src/Form/MigrateUpgradeForm.php b/src/Form/MigrateUpgradeForm.php
index 88c7b37..a9ba852 100644
--- a/src/Form/MigrateUpgradeForm.php
+++ b/src/Form/MigrateUpgradeForm.php
@@ -17,24 +17,30 @@ use Drupal\migrate\Entity\MigrationInterface;
 use Drupal\migrate_upgrade\MigrationCreationTrait;
 
 /**
- * Multi-step form for performing direct site upgrades.
+ * Defines a multi-step form for performing direct site upgrades.
  */
 class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
 
   use MigrationCreationTrait;
 
   /**
-   * The choices of what to do when an upgrade has previously been run.
+   * If a migration has previously run, perform an incremental migration.
    */
   const MIGRATE_UPGRADE_INCREMENTAL = 1;
+
+  /**
+   * If a migration has previously run, roll it back and start fresh.
+   */
   const MIGRATE_UPGRADE_ROLLBACK = 2;
 
   /**
+   * Mapping of known migrations and their source and destination modules.
+   *
    * @todo: Hardcoding this information is not robust - the migrations
    * themselves should hold the necessary information.
    * @see https://www.drupal.org/node/2569805
    *
-   * @var array
+   * @var array[]
    */
   protected $moduleUpgradePaths = [
     'd6_action_settings' => [
@@ -619,7 +625,7 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   public function submitForm(array &$form, FormStateInterface $form_state) {}
 
   /**
-   * Build the form presenting an overview of the migration process.
+   * Builds the form presenting an overview of the migration process.
    *
    * @param array $form
    *   An associative array containing the structure of the form.
@@ -692,7 +698,7 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   }
 
   /**
-   * Overview form submission handler.
+   * Form submission handler for the overview form.
    *
    * @param array $form
    *   An associative array containing the structure of the form.
@@ -715,8 +721,9 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   }
 
   /**
-   * Build the form gathering database credential and file location information.
-   * This is largely borrowed from SiteSettingsForm.
+   * Builds the database credential form and adds file location information.
+   *
+   * This is largely borrowed from \Drupal\Core\Installer\Form\SiteSettingsForm.
    *
    * @param array $form
    *   An associative array containing the structure of the form.
@@ -754,7 +761,7 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
       $form['database']['driver']['#disabled'] = TRUE;
     }
 
-    // Add driver specific configuration options.
+    // Add driver-specific configuration options.
     foreach ($drivers as $key => $driver) {
       $form['database']['driver']['#options'][$key] = $driver->name();
 
@@ -815,7 +822,7 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   }
 
   /**
-   * Credential form validation handler.
+   * Validation handler for the credentials form.
    *
    * @param array $form
    *   An associative array containing the structure of the form.
@@ -896,7 +903,7 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   }
 
   /**
-   * Credential form submission handler.
+   * Submission handler for the credentials form.
    *
    * @param array $form
    *   An associative array containing the structure of the form.
@@ -910,7 +917,7 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   }
 
   /**
-   * Build the form gathering database credential and file location information.
+   * Confirmation form for rollbacks, missing migrations, etc.
    *
    * @param array $form
    *   An associative array containing the structure of the form.
@@ -1041,7 +1048,7 @@ class MigrateUpgradeForm extends FormBase implements ConfirmFormInterface {
   }
 
   /**
-   * Credential form submission handler.
+   * Submission handler for the confirmation form.
    *
    * @param array $form
    *   An associative array containing the structure of the form.
diff --git a/src/MigrateMessageCapture.php b/src/MigrateMessageCapture.php
index 9f622f6..3ccef90 100644
--- a/src/MigrateMessageCapture.php
+++ b/src/MigrateMessageCapture.php
@@ -10,8 +10,7 @@ namespace Drupal\migrate_upgrade;
 use Drupal\migrate\MigrateMessageInterface;
 
 /**
- * Defines a migrate message class for capturing messages rather than
- * displaying them directly.
+ * Allows capturing messages rather than displaying them directly.
  */
 class MigrateMessageCapture implements MigrateMessageInterface {
 
@@ -30,14 +29,14 @@ class MigrateMessageCapture implements MigrateMessageInterface {
   }
 
   /**
-   * Clear out any captured messages.
+   * Clears out any captured messages.
    */
   public function clear() {
     $this->messages = [];
   }
 
   /**
-   * Return any captured messages.
+   * Returns any captured messages.
    *
    * @return array
    */
diff --git a/src/MigrateUpgradeRunBatch.php b/src/MigrateUpgradeRunBatch.php
index 3df43da..667f3b8 100644
--- a/src/MigrateUpgradeRunBatch.php
+++ b/src/MigrateUpgradeRunBatch.php
@@ -54,13 +54,13 @@ class MigrateUpgradeRunBatch {
   protected static $messages;
 
   /**
-   * Run a single migration batch.
+   * Runs a single migration batch.
    *
-   * @param array $initial_ids
+   * @param int[] $initial_ids
    *   The full set of migration IDs to import.
    * @param string $operation
    *   'import' or 'rollback'.
-   * @param $context
+   * @param array $context
    *   The batch context.
    */
   public static function run($initial_ids, $operation, &$context) {
@@ -224,7 +224,7 @@ class MigrateUpgradeRunBatch {
   }
 
   /**
-   * A helper method to grab the logger using the migrate_upgrade channel.
+   * Returns the logger using the migrate_upgrade channel.
    *
    * @return \Psr\Log\LoggerInterface
    *   The logger instance.
@@ -234,16 +234,16 @@ class MigrateUpgradeRunBatch {
   }
 
   /**
-   * Implementation of the Batch API finished method.
+   * Implements the Batch API finished method.
    */
   public static function finished($success, $results, $operations, $elapsed) {
     static::displayResults($results);
   }
 
   /**
-   * Display counts of success/failures on the migration upgrade complete page.
+   * Displays counts of success/failures on the migration upgrade complete page.
    *
-   * @param $results
+   * @param array $results
    *   An array of result data built during the batch.
    */
   protected static function displayResults($results) {
@@ -290,7 +290,7 @@ class MigrateUpgradeRunBatch {
   }
 
   /**
-   * React to item import.
+   * Reacts to item import.
    *
    * @param \Drupal\migrate\Event\MigratePostRowSaveEvent $event
    *   The post-save event.
@@ -303,7 +303,7 @@ class MigrateUpgradeRunBatch {
   }
 
   /**
-   * React to item deletion.
+   * Reacts to item deletion.
    *
    * @param \Drupal\migrate\Event\MigrateRowDeleteEvent $event
    *   The post-save event.
@@ -316,7 +316,7 @@ class MigrateUpgradeRunBatch {
   }
 
   /**
-   * Count up any map save events.
+   * Counts up any map save events.
    *
    * @param \Drupal\migrate\Event\MigrateMapSaveEvent $event
    *   The map event.
@@ -326,7 +326,7 @@ class MigrateUpgradeRunBatch {
   }
 
   /**
-   * Count up any map delete events.
+   * Counts up any map delete events.
    *
    * @param \Drupal\migrate\Event\MigrateMapDeleteEvent $event
    *   The map event.
@@ -336,7 +336,7 @@ class MigrateUpgradeRunBatch {
   }
 
   /**
-   * Display any messages being logged to the ID map.
+   * Displays any messages being logged to the ID map.
    *
    * @param \Drupal\migrate\Event\MigrateIdMapMessageEvent $event
    *   The message event.
diff --git a/src/MigrationCreationTrait.php b/src/MigrationCreationTrait.php
index e808dfd..6e6070a 100644
--- a/src/MigrationCreationTrait.php
+++ b/src/MigrationCreationTrait.php
@@ -15,22 +15,23 @@ use Drupal\migrate\Plugin\RequirementsInterface;
 use Drupal\Component\Plugin\Exception\PluginNotFoundException;
 
 /**
- * Trait providing functionality to create the appropriate migrations for
- * a given source Drupal database. Note the class using the trait must
- * implement TranslationInterface (i.e., define t()).
+ * Creates the appropriate migrations for a given source Drupal database.
+ *
+ * Note the class using the trait must implement
+ * \Drupal\Core\StringTranslation\TranslationInterface (i.e., define t()).
  */
 trait MigrationCreationTrait {
 
   /**
-   * Set up the relevant migrations for import from the provided database
-   * connection.
+   * Sets up the relevant migrations for import from a database connection.
    *
    * @param \Drupal\Core\Database\Database $database
    *   Database array representing the source Drupal database.
    * @param string $source_base_path
    *   Address of the source Drupal site (e.g., http://example.com/).
    *
-   * @return array
+   * @return int[]
+   *   An array of Migration entity IDs.
    */
   protected function createMigrations(array $database, $source_base_path) {
     // Set up the connection.
@@ -94,11 +95,14 @@ trait MigrationCreationTrait {
   }
 
   /**
-   * Determine what version of Drupal the source database contains.
+   * Determines what version of Drupal the source database contains.
    *
    * @param \Drupal\Core\Database\Connection $connection
+   *   The database connection object.
    *
    * @return int|FALSE
+   *   An integer representing the major branch of Drupal core (e.g. '6' for
+   *   Drupal 6.x), or FALSE if no valid version is matched.
    */
   protected function getLegacyDrupalVersion(Connection $connection) {
     // Don't assume because a table of that name exists, that it has the columns