diff -u b/core/modules/migrate/src/Plugin/migrate/process/MigrationLookup.php b/core/modules/migrate/src/Plugin/migrate/process/MigrationLookup.php
--- b/core/modules/migrate/src/Plugin/migrate/process/MigrationLookup.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/MigrationLookup.php
@@ -14,18 +14,18 @@
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
- * Calculates the value of a property based on a previous migration.
+ * Looks up the value of a property based on a previous migration.
  *
  * It is important to maintain relationships among content coming from the
  * source site, but in most instances the unique identifiers of the content
  * change in the process of migration. For example, on the source site a given
  * user account may have an "author" ID of 123, but the Drupal user account
- * created from it may have a uid of 456. The migration_lookup process maintains
- * the relationships between source and destination identifiers in map tables,
- * and this information is leveraged by the migration process plugin.
+ * created from it may have a uid of 456. The migration process maintains the
+ * relationships between source and destination identifiers in map tables, and
+ * this information is leveraged by the migration_lookup process plugin.
  *
  * Available configuration keys
- * - migration: An array of migration ids.
+ * - migration: A single migration id, or an array of migration ids.
  * - source_ids: (optional) An array keyed by migration ids with values that are
  *   a list of source properties.
  * - stub_id: (optional) Identifies which entry in the migration array will be
@@ -58,22 +58,22 @@
  * @code
  * process:
  *   uid:
- *   plugin: migration_lookup
- *   migration:
- *     - users
- *     - members
- *   source_ids:
- *     users:
- *       - author
- *     members:
- *       - author
+ *     plugin: migration_lookup
+ *     migration:
+ *       - users
+ *       - members
+ *     source_ids:
+ *       users:
+ *         - author
+ *       members:
+ *         - author
  * @endcode
  *
- * If the migration_lookup does not find the source ID in the migration map it
- * will create a stub entity for the relationship to use. This stub is generated
- * by the migration provided. In the case of multiple migrations the first value
- * of migration list will be used, but you can select the migration you wish to
- * use to create the stub:
+ * If the migration_lookup plugin does not find the source ID in the migration
+ * map it will create a stub entity for the relationship to use. This stub is
+ * generated by the migration provided. In the case of multiple migrations the
+ * first value of the migration list will be used, but you can select the
+ * migration you wish to* use to create the stub:
  *
  * @code
  * process:
@@ -99,10 +99,10 @@
  * @code
  * process:
  *   uid:
- *   plugin: migration_lookup
- *   migration: users
- *   no_stub: true
- *   source: author
+ *     plugin: migration_lookup
+ *     migration: users
+ *     no_stub: true
+ *     source: author
  * @endcode
  *
  * Scenario:
reverted:
--- b/core/modules/migrate/tests/src/Unit/process/MigrationLookupTest.php
+++ a/core/modules/migrate/tests/src/Unit/process/MigrationTest.php
@@ -4,7 +4,7 @@
 
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\migrate\Plugin\MigrationInterface;
+use Drupal\migrate\Plugin\migrate\process\Migration;
-use Drupal\migrate\Plugin\migrate\process\MigrationLookup;
 use Drupal\migrate\Plugin\MigrateDestinationInterface;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
 use Drupal\migrate\Plugin\MigratePluginManager;
@@ -13,10 +13,10 @@
 use Prophecy\Argument;
 
 /**
+ * @coversDefaultClass \Drupal\migrate\Plugin\migrate\process\Migration
- * @coversDefaultClass \Drupal\migrate\Plugin\migrate\process\MigrationLookup
  * @group migrate
  */
+class MigrationTest extends MigrateProcessTestCase {
-class MigrationLookupTest extends MigrateProcessTestCase {
 
   /**
    * @covers ::transform
@@ -43,7 +43,7 @@
     $migration_plugin->id()->willReturn('actual_migration');
     $destination_migration->getDestinationPlugin(TRUE)->shouldNotBeCalled();
 
+    $migration = new Migration($configuration, '', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
-    $migration = new MigrationLookup($configuration, '', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
     $result = $migration->transform(1, $this->migrateExecutable, $this->row, '');
     $this->assertNull($result);
   }
@@ -82,7 +82,7 @@
     $destination_plugin->import(Argument::any())->willReturn([2]);
     $destination_migration->getDestinationPlugin(TRUE)->willReturn($destination_plugin->reveal());
 
+    $migration = new Migration($configuration, '', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
-    $migration = new MigrationLookup($configuration, '', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
     $result = $migration->transform(1, $this->migrateExecutable, $this->row, '');
     $this->assertEquals(2, $result);
   }
@@ -101,7 +101,7 @@
       'migration' => 'foobaz',
     ];
     $migration_plugin->id()->willReturn(uniqid());
+    $migration = new Migration($configuration, 'migration', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
-    $migration = new MigrationLookup($configuration, 'migration_lookup', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
     $migration->transform(0, $this->migrateExecutable, $this->row, 'foo');
   }
 
@@ -141,7 +141,7 @@
       ->loadMultiple(['foobaz'])
       ->willReturn([$migration_plugin->reveal()]);
 
+    $migration = new Migration($configuration, 'migration', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
-    $migration = new MigrationLookup($configuration, 'migration_lookup', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
     $this->assertSame($expected_value, $migration->transform($source_value, $this->migrateExecutable, $this->row, 'foo'));
   }
 
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/tests/src/Unit/process/MigrationLookupTest.php
@@ -0,0 +1,202 @@
+<?php
+
+namespace Drupal\Tests\migrate\Unit\process;
+
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\migrate\Plugin\MigrationInterface;
+use Drupal\migrate\Plugin\migrate\process\MigrationLookup;
+use Drupal\migrate\Plugin\MigrateDestinationInterface;
+use Drupal\migrate\Plugin\MigrateIdMapInterface;
+use Drupal\migrate\Plugin\MigratePluginManager;
+use Drupal\migrate\Plugin\MigrateSourceInterface;
+use Drupal\migrate\Plugin\MigrationPluginManagerInterface;
+use Prophecy\Argument;
+
+/**
+ * @coversDefaultClass \Drupal\migrate\Plugin\migrate\process\MigrationLookup
+ * @group migrate
+ */
+class MigrationLookupTest extends MigrateProcessTestCase {
+
+  /**
+   * @covers ::transform
+   */
+  public function testTransformWithStubSkipping() {
+    $migration_plugin = $this->prophesize(MigrationInterface::class);
+    $migration_plugin_manager = $this->prophesize(MigrationPluginManagerInterface::class);
+    $process_plugin_manager = $this->prophesize(MigratePluginManager::class);
+
+    $destination_id_map = $this->prophesize(MigrateIdMapInterface::class);
+    $destination_migration = $this->prophesize(MigrationInterface::class);
+    $destination_migration->getIdMap()->willReturn($destination_id_map->reveal());
+    $destination_id_map->lookupDestinationId([1])->willReturn(NULL);
+
+    // Ensure the migration plugin manager returns our migration.
+    $migration_plugin_manager->createInstances(Argument::exact(['destination_migration']))
+      ->willReturn(['destination_migration' => $destination_migration->reveal()]);
+
+    $configuration = [
+      'no_stub' => TRUE,
+      'migration' => 'destination_migration',
+    ];
+
+    $migration_plugin->id()->willReturn('actual_migration');
+    $destination_migration->getDestinationPlugin(TRUE)->shouldNotBeCalled();
+
+    $migration = new MigrationLookup($configuration, '', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
+    $result = $migration->transform(1, $this->migrateExecutable, $this->row, '');
+    $this->assertNull($result);
+  }
+
+  /**
+   * @covers ::transform
+   */
+  public function testTransformWithStubbing() {
+    $migration_plugin = $this->prophesize(MigrationInterface::class);
+    $migration_plugin_manager = $this->prophesize(MigrationPluginManagerInterface::class);
+    $process_plugin_manager = $this->prophesize(MigratePluginManager::class);
+
+    $destination_id_map = $this->prophesize(MigrateIdMapInterface::class);
+    $destination_migration = $this->prophesize('Drupal\migrate\Plugin\Migration');
+    $destination_migration->getIdMap()->willReturn($destination_id_map->reveal());
+    $migration_plugin_manager->createInstances(['destination_migration'])
+      ->willReturn(['destination_migration' => $destination_migration->reveal()]);
+    $destination_id_map->lookupDestinationId([1])->willReturn(NULL);
+    $destination_id_map->saveIdMapping(Argument::any(), Argument::any(), MigrateIdMapInterface::STATUS_NEEDS_UPDATE)->willReturn(NULL);
+
+    $configuration = [
+      'no_stub' => FALSE,
+      'migration' => 'destination_migration',
+    ];
+
+    $migration_plugin->id()->willReturn('actual_migration');
+    $destination_migration->id()->willReturn('destination_migration');
+    $destination_migration->getDestinationPlugin(TRUE)->shouldBeCalled();
+    $destination_migration->getProcess()->willReturn([]);
+    $destination_migration->getSourceConfiguration()->willReturn([]);
+
+    $source_plugin = $this->prophesize(MigrateSourceInterface::class);
+    $source_plugin->getIds()->willReturn(['nid']);
+    $destination_migration->getSourcePlugin()->willReturn($source_plugin->reveal());
+    $destination_plugin = $this->prophesize(MigrateDestinationInterface::class);
+    $destination_plugin->import(Argument::any())->willReturn([2]);
+    $destination_migration->getDestinationPlugin(TRUE)->willReturn($destination_plugin->reveal());
+
+    $migration = new MigrationLookup($configuration, '', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
+    $result = $migration->transform(1, $this->migrateExecutable, $this->row, '');
+    $this->assertEquals(2, $result);
+  }
+
+  /**
+   * Tests that processing is skipped when the input value is empty.
+   *
+   * @expectedException \Drupal\migrate\MigrateSkipProcessException
+   */
+  public function testSkipOnEmpty() {
+    $migration_plugin = $this->prophesize(MigrationInterface::class);
+    $migration_plugin_manager = $this->prophesize(MigrationPluginManagerInterface::class);
+    $process_plugin_manager = $this->prophesize(MigratePluginManager::class);
+
+    $configuration = [
+      'migration' => 'foobaz',
+    ];
+    $migration_plugin->id()->willReturn(uniqid());
+    $migration = new MigrationLookup($configuration, 'migration_lookup', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
+    $migration->transform(0, $this->migrateExecutable, $this->row, 'foo');
+  }
+
+  /**
+   * Tests a successful lookup.
+   *
+   * @dataProvider successfulLookupDataProvider
+   *
+   * @param array $source_id_values
+   *   The source id(s) of the migration map.
+   * @param array $destination_id_values
+   *   The destination id(s) of the migration map.
+   * @param string|array $source_value
+   *   The source value(s) for the migration process plugin.
+   * @param string|array $expected_value
+   *   The expected value(s) of the migration process plugin.
+   */
+  public function testSuccessfulLookup($source_id_values, $destination_id_values, $source_value, $expected_value) {
+    $migration_plugin = $this->prophesize(MigrationInterface::class);
+    $migration_plugin_manager = $this->prophesize(MigrationPluginManagerInterface::class);
+    $process_plugin_manager = $this->prophesize(MigratePluginManager::class);
+
+    $configuration = [
+      'migration' => 'foobaz',
+    ];
+    $migration_plugin->id()->willReturn(uniqid());
+
+    $id_map = $this->prophesize(MigrateIdMapInterface::class);
+    $id_map->lookupDestinationId($source_id_values)->willReturn($destination_id_values);
+    $migration_plugin->getIdMap()->willReturn($id_map->reveal());
+
+    $migration_plugin_manager->createInstances(['foobaz'])
+      ->willReturn(['foobaz' => $migration_plugin->reveal()]);
+
+    $migrationStorage = $this->prophesize(EntityStorageInterface::class);
+    $migrationStorage
+      ->loadMultiple(['foobaz'])
+      ->willReturn([$migration_plugin->reveal()]);
+
+    $migration = new MigrationLookup($configuration, 'migration_lookup', [], $migration_plugin->reveal(), $migration_plugin_manager->reveal(), $process_plugin_manager->reveal());
+    $this->assertSame($expected_value, $migration->transform($source_value, $this->migrateExecutable, $this->row, 'foo'));
+  }
+
+  /**
+   * Provides data for the successful lookup test.
+   *
+   * @return array
+   */
+  public function successfulLookupDataProvider() {
+    return [
+      // Test data for scalar to scalar.
+      [
+        // Source ID of the migration map.
+        [1],
+        // Destination ID of the migration map.
+        [3],
+        // Input value for the migration plugin.
+        1,
+        // Expected output value of the migration plugin.
+        3,
+      ],
+      // Test data for scalar to array.
+      [
+        // Source ID of the migration map.
+        [1],
+        // Destination IDs of the migration map.
+        [3, 'foo'],
+        // Input value for the migration plugin.
+        1,
+        // Expected output values of the migration plugin.
+        [3, 'foo'],
+      ],
+      // Test data for array to scalar.
+      [
+        // Source IDs of the migration map.
+        [1, 3],
+        // Destination ID of the migration map.
+        ['foo'],
+        // Input values for the migration plugin.
+        [1, 3],
+        // Expected output value of the migration plugin.
+        'foo',
+      ],
+      // Test data for array to array.
+      [
+        // Source IDs of the migration map.
+        [1, 3],
+        // Destination IDs of the migration map.
+        [3, 'foo'],
+        // Input values for the migration plugin.
+        [1, 3],
+        // Expected output values of the migration plugin.
+        [3, 'foo'],
+      ],
+    ];
+  }
+
+}