diff --git a/core/modules/aggregator/migration_templates/d6_aggregator_feed.yml b/core/modules/aggregator/migration_templates/d6_aggregator_feed.yml
index cad1553..8689d5b 100644
--- a/core/modules/aggregator/migration_templates/d6_aggregator_feed.yml
+++ b/core/modules/aggregator/migration_templates/d6_aggregator_feed.yml
@@ -1,5 +1,6 @@
 id: d6_aggregator_feed
 label: Aggregator feeds
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/aggregator/migration_templates/d6_aggregator_item.yml b/core/modules/aggregator/migration_templates/d6_aggregator_item.yml
index e14dbd6..7c991eb 100644
--- a/core/modules/aggregator/migration_templates/d6_aggregator_item.yml
+++ b/core/modules/aggregator/migration_templates/d6_aggregator_item.yml
@@ -1,5 +1,6 @@
 id: d6_aggregator_item
 label: Aggregator items
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/aggregator/migration_templates/d7_aggregator_feed.yml b/core/modules/aggregator/migration_templates/d7_aggregator_feed.yml
index 5dbeb25..48eb29d 100644
--- a/core/modules/aggregator/migration_templates/d7_aggregator_feed.yml
+++ b/core/modules/aggregator/migration_templates/d7_aggregator_feed.yml
@@ -1,5 +1,6 @@
 id: d7_aggregator_feed
 label: Aggregator feeds
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/aggregator/migration_templates/d7_aggregator_item.yml b/core/modules/aggregator/migration_templates/d7_aggregator_item.yml
index 054ba43..342c5c8 100644
--- a/core/modules/aggregator/migration_templates/d7_aggregator_item.yml
+++ b/core/modules/aggregator/migration_templates/d7_aggregator_item.yml
@@ -1,5 +1,6 @@
 id: d7_aggregator_item
 label: Aggregator items
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/block_content/migration_templates/d6_custom_block.yml b/core/modules/block_content/migration_templates/d6_custom_block.yml
index 55fbcb5..071e4de 100644
--- a/core/modules/block_content/migration_templates/d6_custom_block.yml
+++ b/core/modules/block_content/migration_templates/d6_custom_block.yml
@@ -1,5 +1,6 @@
 id: d6_custom_block
 label: Custom blocks
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/block_content/migration_templates/d7_custom_block.yml b/core/modules/block_content/migration_templates/d7_custom_block.yml
index ca06cf0..1a9ea19 100644
--- a/core/modules/block_content/migration_templates/d7_custom_block.yml
+++ b/core/modules/block_content/migration_templates/d7_custom_block.yml
@@ -1,5 +1,6 @@
 id: d7_custom_block
 label: Custom blocks
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/comment/migration_templates/d6_comment.yml b/core/modules/comment/migration_templates/d6_comment.yml
index 161820e..afab0cb 100644
--- a/core/modules/comment/migration_templates/d6_comment.yml
+++ b/core/modules/comment/migration_templates/d6_comment.yml
@@ -1,5 +1,6 @@
 id: d6_comment
 label: Comments
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/comment/migration_templates/d7_comment.yml b/core/modules/comment/migration_templates/d7_comment.yml
index dff4b64..0837df9 100644
--- a/core/modules/comment/migration_templates/d7_comment.yml
+++ b/core/modules/comment/migration_templates/d7_comment.yml
@@ -1,5 +1,6 @@
 id: d7_comment
 label: Comments
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/file/migration_templates/d6_file.yml b/core/modules/file/migration_templates/d6_file.yml
index 6544d7d..5b8b672 100644
--- a/core/modules/file/migration_templates/d6_file.yml
+++ b/core/modules/file/migration_templates/d6_file.yml
@@ -2,6 +2,7 @@
 # migration as an optional dependency.
 id: d6_file
 label: Public files
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/file/migration_templates/d7_file.yml b/core/modules/file/migration_templates/d7_file.yml
index b63f13e..7e05a28 100644
--- a/core/modules/file/migration_templates/d7_file.yml
+++ b/core/modules/file/migration_templates/d7_file.yml
@@ -2,6 +2,7 @@
 # migration as an optional dependency.
 id: d7_file
 label: Public files
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/file/migration_templates/d7_file_private.yml b/core/modules/file/migration_templates/d7_file_private.yml
index 197c701..51de533 100644
--- a/core/modules/file/migration_templates/d7_file_private.yml
+++ b/core/modules/file/migration_templates/d7_file_private.yml
@@ -1,5 +1,6 @@
 id: d7_file_private
 label: Private files
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/menu_link_content/migration_templates/d6_menu_links.yml b/core/modules/menu_link_content/migration_templates/d6_menu_links.yml
index 2c8ad4a..e05efee 100644
--- a/core/modules/menu_link_content/migration_templates/d6_menu_links.yml
+++ b/core/modules/menu_link_content/migration_templates/d6_menu_links.yml
@@ -1,5 +1,6 @@
 id: d6_menu_links
 label: Menu links
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/menu_link_content/migration_templates/d7_menu_links.yml b/core/modules/menu_link_content/migration_templates/d7_menu_links.yml
index 200a792..81d4eb8 100644
--- a/core/modules/menu_link_content/migration_templates/d7_menu_links.yml
+++ b/core/modules/menu_link_content/migration_templates/d7_menu_links.yml
@@ -1,5 +1,6 @@
 id: d7_menu_links
 label: Menu links
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/migrate/src/Audit/AuditException.php b/core/modules/migrate/src/Audit/AuditException.php
new file mode 100644
index 0000000..3d122e3
--- /dev/null
+++ b/core/modules/migrate/src/Audit/AuditException.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+use Drupal\migrate\Plugin\MigrationInterface;
+
+/**
+ * Exception thrown when a migration audit fails.
+ */
+class AuditException extends \RuntimeException {
+
+  /**
+   * AuditException constructor.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The migration that caused the exception.
+   * @param string $message
+   *   The reason the audit failed.
+   * @param \Exception $previous
+   *   (optional) The previous exception.
+   */
+  public function __construct(MigrationInterface $migration, $message, \Exception $previous = NULL) {
+    $message = sprintf('Cannot audit migration %s: %s', $migration->id(), $message);
+    parent::__construct($message, 0, $previous);
+  }
+
+}
diff --git a/core/modules/migrate/src/Audit/AuditorInterface.php b/core/modules/migrate/src/Audit/AuditorInterface.php
new file mode 100644
index 0000000..331a807
--- /dev/null
+++ b/core/modules/migrate/src/Audit/AuditorInterface.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+use Drupal\migrate\Plugin\MigrationInterface;
+
+/**
+ * Defines an interface for migration auditors.
+ *
+ * A migration auditor is a class which can examine a migration to determine if
+ * it will cause conflicts with data already existing in the destination system.
+ * What kind of auditing it does, and how it does it, is up to the implementing
+ * class.
+ */
+interface AuditorInterface {
+
+  /**
+   * Audits a migration.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The migration to audit.
+   *
+   * @throws \Drupal\migrate\Audit\AuditException if the audit fails.
+   *
+   * @return array
+   *   If the audit detects no potential conflicts, an empty array is returned.
+   *   Otherwise, the return value will be an array with a single element: a
+   *   string explaining the nature of the conflict, keyed by the migration ID.
+   */
+  public function audit(MigrationInterface $migration);
+
+  /**
+   * Audits a set of migrations.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface[] $migrations
+   *   The migrations to audit.
+   *
+   * @return array
+   *   An array of potential conflict information, keyed by migration ID. See
+   *   audit() for more information.
+   */
+  public function auditMultiple(array $migrations);
+
+}
diff --git a/core/modules/migrate/src/Audit/IdAuditor.php b/core/modules/migrate/src/Audit/IdAuditor.php
new file mode 100644
index 0000000..79cfd74
--- /dev/null
+++ b/core/modules/migrate/src/Audit/IdAuditor.php
@@ -0,0 +1,82 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\migrate\Plugin\MigrationInterface;
+
+/**
+ * Audits migrations that create content entities in the destination system.
+ */
+class IdAuditor implements AuditorInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function audit(MigrationInterface $migration) {
+    $plugin_definition = $migration->getPluginDefinition();
+
+    if (empty($plugin_definition['audit'])) {
+      return [];
+    }
+
+    $interface = MaximumValueInterface::class;
+
+    $destination = $migration->getDestinationPlugin();
+    if (!$destination instanceof MaximumValueInterface) {
+      throw new AuditException($migration, "Destination does not implement $interface");
+    }
+
+    $id_map = $migration->getIdMap();
+    if (!$id_map instanceof MaximumValueInterface) {
+      throw new AuditException($migration, "ID map does not implement $interface");
+    }
+
+    if ($destination->max() > $id_map->max()) {
+      return static::conflict($migration);
+    }
+    return [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function auditMultiple(array $migrations) {
+    $conflicts = [];
+
+    foreach ($migrations as $migration) {
+      $conflicts += $this->audit($migration);
+    }
+    ksort($conflicts);
+    return $conflicts;
+  }
+
+  /**
+   * Builds a conflict array.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The migration that has the conflict.
+   *
+   * @return array
+   *   The conflict. See \Drupal\migrate\Audit\AuditorInterface::audit() for
+   *   information on the format of the return value.
+   */
+  public static function conflict(MigrationInterface $migration) {
+    $base_id = $migration->getBaseId();
+
+    $conflict = [
+      $base_id => $base_id,
+    ];
+
+    $label = $migration->label();
+    if (is_string($label)) {
+      $conflict[$base_id] = $label;
+    }
+    elseif ($label instanceof TranslatableMarkup) {
+      $arguments = $label->getArguments();
+      $conflict[$base_id] = isset($arguments['@label']) ? $arguments['@label'] : $label->render();
+    }
+    return $conflict;
+  }
+
+}
diff --git a/core/modules/migrate/src/Audit/MaximumValueInterface.php b/core/modules/migrate/src/Audit/MaximumValueInterface.php
new file mode 100644
index 0000000..4d6eb16
--- /dev/null
+++ b/core/modules/migrate/src/Audit/MaximumValueInterface.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+/**
+ * Defines an interface for destination and ID maps which track a highest ID.
+ *
+ * When implemented by destination plugins, max() should return the highest ID
+ * of the destination entity type that exists in the system. So, for example,
+ * the entity:node plugin should return the highest node ID that exists,
+ * regardless of whether it was created by a migration.
+ *
+ * When implemented by an ID map, max() should return the highest migrated ID
+ * of the destination entity type.
+ */
+interface MaximumValueInterface {
+
+  /**
+   * Returns the highest ID value tracked by the implementing plugin.
+   *
+   * @return int
+   *   The highest ID.
+   */
+  public function max();
+
+}
diff --git a/core/modules/migrate/src/Plugin/Migration.php b/core/modules/migrate/src/Plugin/Migration.php
index c81d498..fb6aaf2 100644
--- a/core/modules/migrate/src/Plugin/Migration.php
+++ b/core/modules/migrate/src/Plugin/Migration.php
@@ -16,6 +16,13 @@
  * The migration process plugin represents one single migration and acts like a
  * container for the information about a single migration such as the source,
  * process and destination plugins.
+ *
+ * Configuration options:
+ *
+ *   - audit: If set to TRUE, the migration's IDs will be audited. This means
+ *     that, for the entity type this migration creates, if the highest
+ *     destination ID is greater than the highest source ID, a warning will be
+ *     displayed that entities might be overwritten.
  */
 class Migration extends PluginBase implements MigrationInterface, RequirementsInterface, ContainerFactoryPluginInterface {
 
diff --git a/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
index cf3ae1d..f4c26ef 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
@@ -9,6 +9,7 @@
 use Drupal\Core\Field\FieldTypePluginManagerInterface;
 use Drupal\Core\TypedData\TranslatableInterface;
 use Drupal\Core\TypedData\TypedDataInterface;
+use Drupal\migrate\Audit\MaximumValueInterface;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\MigrateException;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
@@ -18,7 +19,7 @@
 /**
  * The destination class for all content entities lacking a specific class.
  */
-class EntityContentBase extends Entity {
+class EntityContentBase extends Entity implements MaximumValueInterface {
 
   /**
    * Entity manager.
@@ -111,12 +112,9 @@ protected function save(ContentEntityInterface $entity, array $old_destination_i
   }
 
   /**
-   * Get whether this destination is for translations.
-   *
-   * @return bool
-   *   Whether this destination is for translations.
+   * {@inheritdoc}
    */
-  protected function isTranslationDestination() {
+  public function isTranslationDestination() {
     return !empty($this->configuration['translations']);
   }
 
@@ -294,4 +292,15 @@ protected function getDefinitionFromEntity($key) {
     ] + $field_definition->getSettings();
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function max() {
+    $query = $this->storage->getQuery()
+      ->sort($this->getKey('id'), 'DESC')
+      ->range(0, 1);
+    $result = $query->execute();
+    return (int) reset($result);
+  }
+
 }
diff --git a/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
index b0db476..336e765 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
@@ -78,4 +78,15 @@ public function getIds() {
     throw new MigrateException('This entity type does not support revisions.');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function max() {
+    $query = $this->storage->getQuery()
+      ->sort($this->getKey('revision'), 'DESC')
+      ->range(0, 1);
+    $result = $query->execute();
+    return (int) reset($result);
+  }
+
 }
diff --git a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
index 6bdd51e..98485e8 100644
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -7,6 +7,7 @@
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\migrate\MigrateMessage;
+use Drupal\migrate\Audit\MaximumValueInterface;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\Event\MigrateIdMapMessageEvent;
 use Drupal\migrate\MigrateException;
@@ -27,7 +28,7 @@
  *
  * @PluginID("sql")
  */
-class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryPluginInterface {
+class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryPluginInterface, MaximumValueInterface {
 
   /**
    * Column name of hashed source id values.
@@ -152,6 +153,8 @@ class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryP
    *   The configuration for the plugin.
    * @param \Drupal\migrate\Plugin\MigrationInterface $migration
    *   The migration to do.
+   * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
+   *   The event dispatcher.
    */
   public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, EventDispatcherInterface $event_dispatcher) {
     parent::__construct($configuration, $plugin_id, $plugin_definition);
@@ -169,7 +172,8 @@ public static function create(ContainerInterface $container, array $configuratio
       $plugin_id,
       $plugin_definition,
       $migration,
-      $container->get('event_dispatcher')
+      $container->get('event_dispatcher'),
+      $container->get('plugin.manager.migration')
     );
   }
 
@@ -925,4 +929,63 @@ public function valid() {
     return $this->currentRow !== FALSE;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function max() {
+    $destination_ids = array_filter(
+      $this->migration->getDestinationPlugin()->getIds(),
+      function (array $id) {
+        return $id['type'] == 'integer';
+      }
+    );
+    if (empty($destination_ids)) {
+      throw new \LogicException('Cannot determine the highest migrated ID without an integer ID column');
+    }
+    $field_name = key($destination_ids);
+
+    $sql_field = $this->destinationIdFields()[$field_name];
+    $migration_id = $this->migration->id();
+
+    // List of mapping tables to look in for the highest ID.
+    $map_tables = [
+      $migration_id => $this->mapTableName(),
+    ];
+
+    // If there's a bundle, it means we have a derived migration and we need to
+    // find all the mapping tables from the related derived migrations.
+    if ($base_id = substr($migration_id, 0, strpos($migration_id, static::DERIVATIVE_SEPARATOR))) {
+      // @TODO Inject the plugin manager as a dependency.
+      /** @var \Drupal\migrate\Plugin\MigrationPluginManagerInterface $migration_manager */
+      $migration_manager = \Drupal::service('plugin.manager.migration');
+
+      $migrations = $migration_manager->getDefinitions();
+      foreach ($migrations as $migration_id => $migration) {
+        if ($migration['id'] == $base_id) {
+          // Get this derived migration's mapping table and add it to the list
+          // of mapping tables to look in for the highest ID.
+          $stub = $migration_manager->createInstance($migration_id);
+          $map_tables[$migration_id] = $stub->getIdMap()->mapTableName();
+        }
+      }
+    }
+
+    // Get the highest id from the list of map tables.
+    $ids = [0];
+    foreach ($map_tables as $map_table) {
+      if (!$this->getDatabase()->schema()->tableExists($map_table)) {
+        break;
+      }
+
+      $query = $this->getDatabase()->select($map_table, 'map')
+        ->fields('map', [$sql_field])
+        ->orderBy($sql_field, 'DESC')
+        ->range(0, 1);
+      $ids[] = $query->execute()->fetchField();
+    }
+
+    // Return the highest of the highest IDs.
+    return max($ids);
+  }
+
 }
diff --git a/core/modules/migrate/tests/src/Kernel/MigrateTestBase.php b/core/modules/migrate/tests/src/Kernel/MigrateTestBase.php
index 51e61d4..7f309b5 100644
--- a/core/modules/migrate/tests/src/Kernel/MigrateTestBase.php
+++ b/core/modules/migrate/tests/src/Kernel/MigrateTestBase.php
@@ -7,7 +7,6 @@
 use Drupal\migrate\MigrateExecutable;
 use Drupal\migrate\MigrateMessageInterface;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
-use Drupal\migrate\Plugin\Migration;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\Row;
 
@@ -47,6 +46,9 @@
    */
   protected $sourceDatabase;
 
+  /**
+   * {@inheritdoc}
+   */
   public static $modules = ['migrate'];
 
   /**
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php b/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php
new file mode 100644
index 0000000..13128fb
--- /dev/null
+++ b/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php
@@ -0,0 +1,195 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal\Kernel\d6;
+
+use Drupal\aggregator\Entity\Feed;
+use Drupal\aggregator\Entity\Item;
+use Drupal\block_content\Entity\BlockContent;
+use Drupal\comment\Entity\Comment;
+use Drupal\file\Entity\File;
+use Drupal\menu_link_content\Entity\MenuLinkContent;
+use Drupal\KernelTests\FileSystemModuleDiscoveryDataProviderTrait;
+use Drupal\migrate\Audit\IdAuditor;
+use Drupal\node\Entity\Node;
+use Drupal\taxonomy\Entity\Term;
+use Drupal\user\Entity\User;
+
+/**
+ * Tests the migration auditor for ID conflicts.
+ *
+ * @group migrate_drupal
+ */
+class MigrateDrupal6AuditIdsTest extends MigrateDrupal6TestBase {
+
+  use FileSystemModuleDiscoveryDataProviderTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    // Enable all modules.
+    self::$modules = array_keys($this->coreModuleListDataProvider());
+    parent::setUp();
+
+    // Install required schemas.
+    $this->installEntitySchema('aggregator_item');
+    $this->installEntitySchema('aggregator_feed');
+    $this->installEntitySchema('block_content');
+    $this->installEntitySchema('comment');
+    $this->installEntitySchema('file');
+    $this->installEntitySchema('menu_link_content');
+    $this->installEntitySchema('node');
+    $this->installEntitySchema('taxonomy_term');
+    $this->installSchema('book', ['book']);
+    $this->installSchema('dblog', ['watchdog']);
+    $this->installSchema('forum', ['forum_index']);
+    $this->installSchema('search', ['search_dataset']);
+    $this->installSchema('system', ['sequences']);
+    $this->installSchema('tracker', ['tracker_node', 'tracker_user']);
+  }
+
+  /**
+   * Tests multiple migrations to the same destination with no ID conflicts.
+   */
+  public function testMultipleMigrationWithoutIdConflicts() {
+    // Create a node of type page.
+    Node::create(['type' => 'page', 'title' => 'foo'])->save();
+
+    // Insert data in the d6_node:page migration mappping table to simulate a
+    // previously migrated node.
+    $table_name = $this->getMigration('d6_node:page')->getIdMap()->mapTableName();
+    $this->container->get('database')->insert($table_name)
+      ->fields([
+        'source_ids_hash' => 1,
+        'sourceid1' => 1,
+        'destid1' => 1,
+      ])
+      ->execute();
+
+    // Audit the IDs of the d6_node migrations for the page & article node type.
+    // There should be no conflicts since the highest destination ID should be
+    // equal to the highest migrated ID, as found in the aggregated mapping
+    // tables of the two node migrations.
+    $migrations = [
+      $this->getMigration('d6_node:page'),
+      $this->getMigration('d6_node:article'),
+    ];
+    $this->assertEmpty((new IdAuditor())->auditMultiple($migrations));
+  }
+
+  /**
+   * Tests all migrations with no ID conflicts.
+   */
+  public function testAllMigrationsWithNoIdConflicts() {
+    $migrations = $this->container
+      ->get('plugin.manager.migration')
+      ->createInstancesByTag('Drupal 6');
+
+    // Audit all Drupal 6 migrations that support it. There should be no
+    // conflicts since no content has been created.
+    $this->assertEmpty((new IdAuditor())->auditMultiple($migrations));
+  }
+
+  /**
+   * Tests all migrations with ID conflicts.
+   */
+  public function testAllMigrationsWithIdConflicts() {
+    // Get all Drupal 6 migrations.
+    $migrations = $this->container
+      ->get('plugin.manager.migration')
+      ->createInstancesByTag('Drupal 6');
+
+    // Create content.
+    $this->createContent();
+
+    // Audit the IDs of all migrations. There should be conflicts since content
+    // has been created.
+    $conflicts = (new IdAuditor())->auditMultiple($migrations);
+
+    $expected = [
+      'd6_aggregator_feed',
+      'd6_aggregator_item',
+      'd6_comment',
+      'd6_custom_block',
+      'd6_file',
+      'd6_menu_links',
+      'd6_node',
+      'd6_node_revision',
+      'd6_taxonomy_term',
+      'd6_term_node_revision',
+      'd6_user',
+    ];
+
+    $this->assertSame($expected, array_keys($conflicts));
+  }
+
+  protected function createContent() {
+    // Create an aggregator feed.
+    $feed = Feed::create([
+      'title' => 'feed',
+      'url' => 'http://www.example.com',
+    ]);
+    $feed->save();
+
+    // Create an aggregator feed item.
+    $item = Item::create([
+      'title' => 'feed item',
+      'fid' => $feed->id(),
+      'link' => 'http://www.example.com',
+    ]);
+    $item->save();
+
+    // Create a block content.
+    $block = BlockContent::create([
+      'info' => 'block',
+      'type' => 'block',
+    ]);
+    $block->save();
+
+    // Create a node.
+    $node = Node::create([
+      'type' => 'page',
+      'title' => 'page',
+    ]);
+    $node->save();
+
+    // Create a comment.
+    $comment = Comment::create([
+      'comment_type' => 'comment',
+      'field_name' => 'comment',
+      'entity_type' => 'node',
+      'entity_id' => $node->id(),
+    ]);
+    $comment->save();
+
+    // Create a file.
+    $file = File::create([
+      'uri' => 'public://example.txt',
+    ]);
+    $file->save();
+
+    // Create a menu link.
+    $menu_link = MenuLinkContent::create([
+      'title' => 'menu link',
+      'link' => ['uri' => 'http://www.example.com'],
+      'menu_name' => 'tools',
+    ]);
+    $menu_link->save();
+
+    // Create a taxonomy term.
+    $term = Term::create([
+      'name' => 'term',
+      'vid' => 'term',
+    ]);
+    $term->save();
+
+    // Create a user.
+    $user = User::create([
+      'uid' => 2,
+      'name' => 'user',
+      'mail' => 'user@example.com',
+    ]);
+    $user->save();
+  }
+
+}
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php b/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php
new file mode 100644
index 0000000..b8d4f40
--- /dev/null
+++ b/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php
@@ -0,0 +1,194 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal\Kernel\d7;
+
+use Drupal\aggregator\Entity\Feed;
+use Drupal\aggregator\Entity\Item;
+use Drupal\block_content\Entity\BlockContent;
+use Drupal\comment\Entity\Comment;
+use Drupal\file\Entity\File;
+use Drupal\menu_link_content\Entity\MenuLinkContent;
+use Drupal\KernelTests\FileSystemModuleDiscoveryDataProviderTrait;
+use Drupal\migrate\Audit\IdAuditor;
+use Drupal\node\Entity\Node;
+use Drupal\taxonomy\Entity\Term;
+use Drupal\user\Entity\User;
+
+/**
+ * Tests the migration auditor for ID conflicts.
+ *
+ * @group migrate_drupal
+ */
+class MigrateDrupal7AuditIdsTest extends MigrateDrupal7TestBase {
+
+  use FileSystemModuleDiscoveryDataProviderTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    // Enable all modules.
+    self::$modules = array_keys($this->coreModuleListDataProvider());
+    parent::setUp();
+
+    // Install required schemas.
+    $this->installEntitySchema('aggregator_item');
+    $this->installEntitySchema('aggregator_feed');
+    $this->installEntitySchema('block_content');
+    $this->installEntitySchema('comment');
+    $this->installEntitySchema('file');
+    $this->installEntitySchema('menu_link_content');
+    $this->installEntitySchema('node');
+    $this->installEntitySchema('taxonomy_term');
+    $this->installSchema('book', ['book']);
+    $this->installSchema('dblog', ['watchdog']);
+    $this->installSchema('forum', ['forum_index']);
+    $this->installSchema('search', ['search_dataset']);
+    $this->installSchema('system', ['sequences']);
+    $this->installSchema('tracker', ['tracker_node', 'tracker_user']);
+  }
+
+  /**
+   * Tests multiple migrations to the same destination with no ID conflicts.
+   */
+  public function testMultipleMigrationWithoutIdConflicts() {
+    // Create a node of type page.
+    Node::create(['type' => 'page', 'title' => 'foo'])->save();
+
+    // Insert data in the d7_node:page migration mappping table to simulate a
+    // previously migrated node.
+    $table_name = $this->getMigration('d7_node:page')->getIdMap()->mapTableName();
+    $this->container->get('database')->insert($table_name)
+      ->fields([
+        'source_ids_hash' => 1,
+        'sourceid1' => 1,
+        'destid1' => 1,
+      ])
+      ->execute();
+
+    // Audit the IDs of the d7_node migrations for the page & article node type.
+    // There should be no conflicts since the highest destination ID should be
+    // equal to the highest migrated ID, as found in the aggregated mapping
+    // tables of the two node migrations.
+    $migrations = [
+      $this->getMigration('d7_node:page'),
+      $this->getMigration('d7_node:article'),
+    ];
+    $this->assertEmpty((new IdAuditor())->auditMultiple($migrations));
+  }
+
+  /**
+   * Tests all migrations with no ID conflicts.
+   */
+  public function testAllMigrationsWithNoIdConflicts() {
+    $migrations = $this->container
+      ->get('plugin.manager.migration')
+      ->createInstancesByTag('Drupal 7');
+
+    // Audit the IDs of all Drupal 7 migrations. There should be no conflicts
+    // since no content has been created.
+    $this->assertEmpty((new IdAuditor())->auditMultiple($migrations));
+  }
+
+  /**
+   * Tests all migrations with ID conflicts.
+   */
+  public function testAllMigrationsWithIdConflicts() {
+    $migrations = $this->container
+      ->get('plugin.manager.migration')
+      ->createInstancesByTag('Drupal 7');
+
+    // Create content.
+    $this->createContent();
+
+    // Audit the IDs of all Drupal 7 migrations. There should be conflicts since
+    // content has been created.
+    $conflicts = (new IdAuditor())->auditMultiple($migrations);
+
+    $expected = [
+      'd7_aggregator_feed',
+      'd7_aggregator_item',
+      'd7_comment',
+      'd7_custom_block',
+      'd7_file',
+      'd7_file_private',
+      'd7_menu_links',
+      'd7_node',
+      'd7_node_revision',
+      'd7_taxonomy_term',
+      'd7_user',
+    ];
+
+    $this->assertSame($expected, array_keys($conflicts));
+  }
+
+  protected function createContent() {
+    // Create an aggregator feed.
+    $feed = Feed::create([
+      'title' => 'feed',
+      'url' => 'http://www.example.com',
+    ]);
+    $feed->save();
+
+    // Create an aggregator feed item.
+    $item = Item::create([
+      'title' => 'feed item',
+      'fid' => $feed->id(),
+      'link' => 'http://www.example.com',
+    ]);
+    $item->save();
+
+    // Create a block content.
+    $block = BlockContent::create([
+      'info' => 'block',
+      'type' => 'block',
+    ]);
+    $block->save();
+
+    // Create a node.
+    $node = Node::create([
+      'type' => 'page',
+      'title' => 'page',
+    ]);
+    $node->save();
+
+    // Create a comment.
+    $comment = Comment::create([
+      'comment_type' => 'comment',
+      'field_name' => 'comment',
+      'entity_type' => 'node',
+      'entity_id' => $node->id(),
+    ]);
+    $comment->save();
+
+    // Create a file.
+    $file = File::create([
+      'uri' => 'public://example.txt',
+    ]);
+    $file->save();
+
+    // Create a menu link.
+    $menu_link = MenuLinkContent::create([
+      'title' => 'menu link',
+      'link' => ['uri' => 'http://www.example.com'],
+      'menu_name' => 'tools',
+    ]);
+    $menu_link->save();
+
+    // Create a taxonomy term.
+    $term = Term::create([
+      'name' => 'term',
+      'vid' => 'term',
+    ]);
+    $term->save();
+
+    // Create a user.
+    $user = User::create([
+      'uid' => 2,
+      'name' => 'user',
+      'mail' => 'user@example.com',
+    ]);
+    $user->save();
+  }
+
+}
diff --git a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
index d747e0a..784281f 100644
--- a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
@@ -8,6 +8,8 @@
 use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\State\StateInterface;
 use Drupal\Core\Url;
+use Drupal\migrate\Audit\IdAuditor;
+use Drupal\migrate\Plugin\migrate\destination\EntityContentBase;
 use Drupal\migrate\Plugin\MigrationPluginManagerInterface;
 use Drupal\migrate_drupal_ui\Batch\MigrateUpgradeImportBatch;
 use Drupal\migrate_drupal\MigrationConfigurationTrait;
@@ -100,6 +102,9 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       case 'credentials':
         return $this->buildCredentialForm($form, $form_state);
 
+      case 'confirm_id_conflicts':
+        return $this->buildIdConflictForm($form, $form_state);
+
       case 'confirm':
         return $this->buildConfirmForm($form, $form_state);
 
@@ -434,6 +439,125 @@ public function validateCredentialForm(array &$form, FormStateInterface $form_st
    */
   public function submitCredentialForm(array &$form, FormStateInterface $form_state) {
     // Indicate the next step is confirmation.
+    $form_state->set('step', 'confirm_id_conflicts');
+    $form_state->setRebuild();
+  }
+
+  /**
+   * Confirmation form for ID conflicts.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   *
+   * @return array
+   *   The form structure.
+   */
+  public function buildIdConflictForm(array &$form, FormStateInterface $form_state) {
+    // Check if there are conflicts. If none, just skip this form!
+    $migration_ids = array_keys($form_state->get('migrations'));
+    $migrations = $this->pluginManager->createInstances($migration_ids);
+
+    $auditor = new IdAuditor();
+    $translated_content_conflicts = $content_conflicts = [];
+    foreach ($migrations as $migration) {
+      $audit = $auditor->audit($migration);
+      $destination_plugin = $migration->getDestinationPlugin();
+
+      if ($destination_plugin instanceof EntityContentBase && $destination_plugin->isTranslationDestination()) {
+        $translated_content_conflicts += $audit;
+      }
+      else {
+        $content_conflicts += $audit;
+      }
+    }
+    if (empty($content_conflicts) && empty($translated_content_conflicts)) {
+      $form_state->set('step', 'confirm');
+      return $this->buildForm($form, $form_state);
+    }
+
+    $form['warning'] = [
+      '#type' => 'markup',
+      '#markup' => '<h3>' . $this->t('Content may be overwritten') . '</h3>',
+    ];
+    $form = parent::buildForm($form, $form_state);
+    $form['actions']['submit']['#submit'] = ['::submitConfirmIdConflictForm'];
+    $form['actions']['submit']['#value'] = $this->t('I acknowledge I may lose data, continue anyway.');
+
+    $form = $this->conflictsForm($form, $form_state, $content_conflicts);
+    return $this->i18nWarningForm($form, $form_state, $translated_content_conflicts);
+  }
+
+  /**
+   * Build the markup for conflict warnings.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param array $content_conflicts
+   *   The conflicted migration labels.
+   *
+   * @return array
+   *   The form structure.
+   */
+  protected function conflictsForm(array &$form, FormStateInterface $form_state, $content_conflicts) {
+    if (empty($content_conflicts)) {
+      return $form;
+    }
+
+    $form['warning']['#markup'] .= '<p>' . $this->t('Upgrades work on clean and empty new installs of Drupal 8. However, it looks like you have content in your site that is unknown to the migrate system; perhaps manually added. These new entities <strong>may be overwritten</strong> if you run this upgrade. For more information, see the online handbook entry for <a target="_blank" href=":id-conflicts-handbook">handling migration conflicts</a>.', [':id-conflicts-handbook' => 'https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#id_conflicts']) . '</p>';
+
+    sort($content_conflicts);
+    $form['conflicts'] = [
+      '#title' => $this->t('The conflicting content are of the following types:'),
+      '#theme' => 'item_list',
+      '#items' => $content_conflicts,
+    ];
+
+    return $form;
+  }
+
+  /**
+   * Build the markup for i18n warnings.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param array $translated_content_conflicts
+   *   The conflicted translated migration labels.
+   *
+   * @return array
+   *   The form structure.
+   */
+  protected function i18nWarningForm(array &$form, FormStateInterface $form_state, $translated_content_conflicts) {
+    if (empty($translated_content_conflicts)) {
+      return $form;
+    }
+
+    $form['warning']['#markup'] .= '<p>' . $this->t('It looks like you are migrating translated content. Be extra cautious and make sure you aren\'t introducting any conflicts. For more on the subject, see the online handbook entry for <a target="_blank" href=":id-conflicts-handbook">handling migration conflicts</a>.', [':id-conflicts-handbook' => 'https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#id_conflicts']) . '</p>';
+
+    sort($translated_content_conflicts);
+    $form['i18n'] = [
+      '#title' => $this->t('The translated content are of the following types:'),
+      '#theme' => 'item_list',
+      '#items' => $translated_content_conflicts,
+    ];
+
+    return $form;
+  }
+
+  /**
+   * Submission handler for the confirmation form.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   */
+  public function submitConfirmIdConflictForm(array &$form, FormStateInterface $form_state) {
     $form_state->set('step', 'confirm');
     $form_state->setRebuild();
   }
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
index e322aa3..46d8303 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -152,8 +152,10 @@ public function testMigrateUpgrade() {
 
     $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
     $this->assertResponse(200);
-    $this->assertText('Are you sure?');
+    $this->assertSession()->pageTextContains('Content may be overwritten');
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
     // Ensure we get errors about missing modules.
+    $this->assertSession()->pageTextContains('Upgrade analysis report');
     $this->assertSession()->pageTextContains(t('Source module not found for migration_provider_no_annotation.'));
     $this->assertSession()->pageTextContains(t('Source module not found for migration_provider_test.'));
     $this->assertSession()->pageTextContains(t('Destination module not found for migration_provider_test'));
@@ -171,8 +173,10 @@ public function testMigrateUpgrade() {
 
     $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
     $this->assertSession()->statusCodeEquals(200);
-    $this->assertSession()->pageTextContains('Are you sure?');
+    $this->assertSession()->pageTextContains('Content may be overwritten');
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
     // Ensure there are no errors about the missing modules from the test module.
+    $this->assertSession()->pageTextContains('Upgrade analysis report');
     $this->assertSession()->pageTextNotContains(t('Source module not found for migration_provider_no_annotation.'));
     $this->assertSession()->pageTextNotContains(t('Source module not found for migration_provider_test.'));
     $this->assertSession()->pageTextNotContains(t('Destination module not found for migration_provider_test'));
diff --git a/core/modules/node/migration_templates/d6_node.yml b/core/modules/node/migration_templates/d6_node.yml
index 56d0459..84a4bf1 100644
--- a/core/modules/node/migration_templates/d6_node.yml
+++ b/core/modules/node/migration_templates/d6_node.yml
@@ -1,5 +1,6 @@
 id: d6_node
 label: Nodes
+audit: true
 migration_tags:
   - Drupal 6
 deriver: Drupal\node\Plugin\migrate\D6NodeDeriver
diff --git a/core/modules/node/migration_templates/d6_node_revision.yml b/core/modules/node/migration_templates/d6_node_revision.yml
index f4ff301..74a42d4 100644
--- a/core/modules/node/migration_templates/d6_node_revision.yml
+++ b/core/modules/node/migration_templates/d6_node_revision.yml
@@ -1,5 +1,6 @@
 id: d6_node_revision
 label: Node revisions
+audit: true
 migration_tags:
   - Drupal 6
 deriver: Drupal\node\Plugin\migrate\D6NodeDeriver
diff --git a/core/modules/node/migration_templates/d7_node.yml b/core/modules/node/migration_templates/d7_node.yml
index 359be81..8036797 100644
--- a/core/modules/node/migration_templates/d7_node.yml
+++ b/core/modules/node/migration_templates/d7_node.yml
@@ -1,5 +1,6 @@
 id: d7_node
 label: Nodes
+audit: true
 migration_tags:
   - Drupal 7
 deriver: Drupal\node\Plugin\migrate\D7NodeDeriver
diff --git a/core/modules/node/migration_templates/d7_node_revision.yml b/core/modules/node/migration_templates/d7_node_revision.yml
index c6081ef..18c90b6 100644
--- a/core/modules/node/migration_templates/d7_node_revision.yml
+++ b/core/modules/node/migration_templates/d7_node_revision.yml
@@ -1,5 +1,6 @@
 id: d7_node_revision
 label: Node revisions
+audit: true
 migration_tags:
   - Drupal 7
 deriver: Drupal\node\Plugin\migrate\D7NodeDeriver
diff --git a/core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml b/core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml
index e3c3e3d..9eafee5 100644
--- a/core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml
+++ b/core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml
@@ -1,5 +1,6 @@
 id: d6_taxonomy_term
 label: Taxonomy terms
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/taxonomy/migration_templates/d6_term_node_revision.yml b/core/modules/taxonomy/migration_templates/d6_term_node_revision.yml
index 91c8362..c3ebe30 100644
--- a/core/modules/taxonomy/migration_templates/d6_term_node_revision.yml
+++ b/core/modules/taxonomy/migration_templates/d6_term_node_revision.yml
@@ -1,5 +1,6 @@
 id: d6_term_node_revision
 label: Term/node relationship revisions
+audit: true
 migration_tags:
   - Drupal 6
 deriver: Drupal\taxonomy\Plugin\migrate\D6TermNodeDeriver
diff --git a/core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml b/core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml
index 46f9f20..6033d4f 100644
--- a/core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml
+++ b/core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml
@@ -1,5 +1,6 @@
 id: d7_taxonomy_term
 label: Taxonomy terms
+audit: true
 migration_tags:
   - Drupal 7
 deriver: Drupal\taxonomy\Plugin\migrate\D7TaxonomyTermDeriver
diff --git a/core/modules/user/migration_templates/d6_user.yml b/core/modules/user/migration_templates/d6_user.yml
index d58607b..35d31da 100644
--- a/core/modules/user/migration_templates/d6_user.yml
+++ b/core/modules/user/migration_templates/d6_user.yml
@@ -1,5 +1,6 @@
 id: d6_user
 label: User accounts
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/user/migration_templates/d7_user.yml b/core/modules/user/migration_templates/d7_user.yml
index 54c8805..c93288f 100644
--- a/core/modules/user/migration_templates/d7_user.yml
+++ b/core/modules/user/migration_templates/d7_user.yml
@@ -1,8 +1,9 @@
 id: d7_user
 label: User accounts
+audit: true
 migration_tags:
   - Drupal 7
-class: Drupal\user\Plugin\migrate\User
+class: \Drupal\user\Plugin\migrate\User
 source:
   plugin: d7_user
 process:
diff --git a/core/modules/user/src/Plugin/migrate/destination/EntityUser.php b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
index 7c2c81a..aceb86d 100644
--- a/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
+++ b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
@@ -127,4 +127,18 @@ protected function processStubRow(Row $row) {
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function max() {
+    $highest_id = parent::max();
+
+    // Every Drupal site must have a user with UID of 1 and it's normal for
+    // migrations to overwrite this user.
+    if ($highest_id == 1) {
+      return 0;
+    }
+    return $highest_id;
+  }
+
 }