diff --git a/core/config/schema/core.data_types.schema.yml b/core/config/schema/core.data_types.schema.yml
index 73db361..99954c4 100644
--- a/core/config/schema/core.data_types.schema.yml
+++ b/core/config/schema/core.data_types.schema.yml
@@ -46,7 +46,7 @@ mapping:
 sequence:
   label: Sequence
   class: '\Drupal\Core\Config\Schema\Sequence'
-  definition_class: '\Drupal\Core\TypedData\ListDataDefinition'
+  definition_class: '\Drupal\Core\TypedData\MapDataDefinition'
 
 # Simple extended data types:
 
diff --git a/core/lib/Drupal/Core/Config/Schema/ArrayElement.php b/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
index 7fc6cc5..4c11305 100644
--- a/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
+++ b/core/lib/Drupal/Core/Config/Schema/ArrayElement.php
@@ -2,10 +2,12 @@
 
 namespace Drupal\Core\Config\Schema;
 
+use Drupal\Core\TypedData\ComplexDataInterface;
+
 /**
  * Defines a generic configuration element that contains multiple properties.
  */
-abstract class ArrayElement extends Element implements \IteratorAggregate, TypedConfigInterface {
+abstract class ArrayElement extends Element implements \IteratorAggregate, TypedConfigInterface, ComplexDataInterface {
 
   /**
    * Parsed elements.
@@ -161,4 +163,28 @@ public function isNullable() {
     return isset($this->definition['nullable']) && $this->definition['nullable'] == TRUE;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function set($property_name, $value, $notify = TRUE) {
+    $this->value[$property_name] = $value;
+    // Config schema elements to not make use of notifications. Thus, we skip
+    // notifying parents.
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getProperties($include_computed = FALSE) {
+    $properties = array();
+    foreach (array_keys($this->value) as $name) {
+      $definition = $this->getElementDefinition($name);
+      if ($include_computed || !$definition->isComputed()) {
+        $properties[$name] = $this->get($name);
+      }
+    }
+    return $properties;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Config/Schema/Mapping.php b/core/lib/Drupal/Core/Config/Schema/Mapping.php
index 44af54d..2f4b58f 100644
--- a/core/lib/Drupal/Core/Config/Schema/Mapping.php
+++ b/core/lib/Drupal/Core/Config/Schema/Mapping.php
@@ -2,8 +2,6 @@
 
 namespace Drupal\Core\Config\Schema;
 
-use Drupal\Core\TypedData\ComplexDataInterface;
-
 /**
  * Defines a mapping configuration element.
  *
@@ -17,7 +15,7 @@
  * Read https://www.drupal.org/node/1905070 for more details about configuration
  * schema, types and type resolution.
  */
-class Mapping extends ArrayElement implements ComplexDataInterface {
+class Mapping extends ArrayElement {
 
   /**
    * {@inheritdoc}
@@ -28,24 +26,4 @@ protected function getElementDefinition($key) {
     return $this->buildDataDefinition($definition, $value, $key);
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function set($property_name, $value, $notify = TRUE) {
-    $this->value[$property_name] = $value;
-    // @todo notify?
-    return $this;
-  }
-
-  public function getProperties($include_computed = FALSE) {
-    $properties = array();
-    foreach (array_keys($this->value) as $name) {
-      $definition = $this->getElementDefinition($name);
-      if ($include_computed || !$definition->isComputed()) {
-        $properties[$name] = $this->get($name);
-      }
-    }
-    return $properties;
-  }
-
 }
diff --git a/core/lib/Drupal/Core/Config/Schema/Sequence.php b/core/lib/Drupal/Core/Config/Schema/Sequence.php
index 476f8aa..79e6f9f 100644
--- a/core/lib/Drupal/Core/Config/Schema/Sequence.php
+++ b/core/lib/Drupal/Core/Config/Schema/Sequence.php
@@ -2,8 +2,6 @@
 
 namespace Drupal\Core\Config\Schema;
 
-use Drupal\Core\TypedData\TraversableTypedDataInterface;
-
 /**
  * Defines a configuration element of type Sequence.
  *
@@ -12,8 +10,14 @@
  *
  * Read https://www.drupal.org/node/1905070 for more details about configuration
  * schema, types and type resolution.
+ *
+ * Note that sequences implement the typed data ComplexDataInterface (via the
+ * parent ArrayElement) rather than the ListInterface. This is, as sequences
+ * have named keys, what is not covered by lists. From the typed data API
+ * perspective sequences are handled as ordered mappings without metadata about
+ * existing properties.
  */
-class Sequence extends ArrayElement implements TraversableTypedDataInterface {
+class Sequence extends ArrayElement {
 
   /**
    * {@inheritdoc}
diff --git a/core/lib/Drupal/Core/Field/BaseFieldDefinition.php b/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
index f2600c7..594f31d 100644
--- a/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
+++ b/core/lib/Drupal/Core/Field/BaseFieldDefinition.php
@@ -576,6 +576,16 @@ protected function getFieldItemClass() {
   /**
    * {@inheritdoc}
    */
+  public function __sleep() {
+    // Do not serialize the statically cached property definitions.
+    $vars = get_object_vars($this);
+    unset($vars['propertyDefinitions']);
+    return array_keys($vars);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function getTargetEntityTypeId() {
     return isset($this->definition['entity_type']) ? $this->definition['entity_type'] : NULL;
   }
diff --git a/core/lib/Drupal/Core/TypedData/ComplexDataDefinitionBase.php b/core/lib/Drupal/Core/TypedData/ComplexDataDefinitionBase.php
index 95ba817..0acdba4 100644
--- a/core/lib/Drupal/Core/TypedData/ComplexDataDefinitionBase.php
+++ b/core/lib/Drupal/Core/TypedData/ComplexDataDefinitionBase.php
@@ -36,4 +36,14 @@ public function getMainPropertyName() {
     return NULL;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function __sleep() {
+    // Do not serialize the cached property definitions.
+    $vars = get_object_vars($this);
+    unset($vars['propertyDefinitions']);
+    return array_keys($vars);
+  }
+
 }
diff --git a/core/lib/Drupal/Core/TypedData/DataDefinition.php b/core/lib/Drupal/Core/TypedData/DataDefinition.php
index 866beb3..07b78bf 100644
--- a/core/lib/Drupal/Core/TypedData/DataDefinition.php
+++ b/core/lib/Drupal/Core/TypedData/DataDefinition.php
@@ -345,14 +345,4 @@ public function toArray() {
     return $this->definition;
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function __sleep() {
-    // Do not serialize the statically cached property definitions.
-    $vars = get_object_vars($this);
-    unset($vars['propertyDefinitions'], $vars['typedDataManager']);
-    return array_keys($vars);
-  }
-
 }
diff --git a/core/lib/Drupal/Core/TypedData/Validation/RecursiveContextualValidator.php b/core/lib/Drupal/Core/TypedData/Validation/RecursiveContextualValidator.php
index c52c9db..b765b05 100644
--- a/core/lib/Drupal/Core/TypedData/Validation/RecursiveContextualValidator.php
+++ b/core/lib/Drupal/Core/TypedData/Validation/RecursiveContextualValidator.php
@@ -4,7 +4,6 @@
 
 use Drupal\Core\TypedData\ComplexDataInterface;
 use Drupal\Core\TypedData\ListInterface;
-use Drupal\Core\TypedData\TraversableTypedDataInterface;
 use Drupal\Core\TypedData\TypedDataInterface;
 use Drupal\Core\TypedData\TypedDataManagerInterface;
 use Symfony\Component\Validator\Constraint;
@@ -143,10 +142,7 @@ protected function validateNode(TypedDataInterface $data, $constraints = NULL, $
 
     // If the data is a list or complex data, validate the contained list items
     // or properties. However, do not recurse if the data is empty.
-    // Note: Most traversable, doesn't have a concept of empty/not empty, but
-    // most implementations, like ListInterface/TypedConfigInterface have.
-    // Therefor first check, that the method exists.
-    if ($data instanceof TraversableTypedDataInterface && (!(method_exists($data, 'isEmpty') && $data->isEmpty()))) {
+    if (($data instanceof ListInterface || $data instanceof ComplexDataInterface) && !$data->isEmpty()) {
       foreach ($data as $name => $property) {
         $this->validateNode($property);
       }
diff --git a/core/tests/Drupal/KernelTests/Config/ConfigValidationTest.php b/core/tests/Drupal/KernelTests/Config/ConfigValidationTest.php
deleted file mode 100644
index af6a4c5..0000000
--- a/core/tests/Drupal/KernelTests/Config/ConfigValidationTest.php
+++ /dev/null
@@ -1,79 +0,0 @@
-<?php
-
-namespace Drupal\KernelTests\Config;
-
-use Drupal\KernelTests\KernelTestBase;
-use Symfony\Component\Validator\ConstraintViolationListInterface;
-
-/**
- * Tests config validation mechanism.
- *
- * @group Config
- */
-class ConfigValidationTest extends KernelTestBase {
-
-  /**
-   * {@inheritdoc}
-   */
-  public static $modules = ['config_test'];
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function setUp() {
-    parent::setUp();
-
-    $this->installConfig('config_test');
-  }
-
-  public function testSimpleConfigValidation() {
-    $config = \Drupal::configFactory()->getEditable('config_test.validation');
-    /** @var \Drupal\Core\Config\TypedConfigManagerInterface $typed_config_manager */
-    $typed_config_manager = \Drupal::service('config.typed');
-    /** @var \Drupal\Core\Config\Schema\TypedConfigInterface $typed_config */
-    $typed_config = $typed_config_manager->get('config_test.validation');
-
-    $result = $typed_config->validate();
-    $this->assertInstanceOf(ConstraintViolationListInterface::class, $result);
-    $this->assertEmpty($result);
-
-    // Test constraints on primitive types.
-    $config->set('llama', 'muh');
-    $config->save();
-
-    $typed_config = $typed_config_manager->get('config_test.validation');
-    $result = $typed_config->validate();
-    // Its not a valid llama anymore.
-    $this->assertCount(1, $result);
-    $this->assertEquals('no valid llama', $result->get(0)->getMessage());
-
-    // Test constraints on mapping.
-    $config->set('llama', 'meh');
-    $config->set('cat.type', 'nyans');
-    $config->save();
-
-    $typed_config = $typed_config_manager->get('config_test.validation');
-    $result = $typed_config->validate();
-    $this->assertEmpty($result);
-
-    // Test constrains on nested mapping.
-    $config->set('cat.type', 'miaus');
-    $config->save();
-
-    $typed_config = $typed_config_manager->get('config_test.validation');
-    $result = $typed_config->validate();
-    $this->assertCount(1, $result);
-    $this->assertEquals('no valid cat', $result->get(0)->getMessage());
-
-    // Test constrains on sequences.
-    $config->set('cat.type', 'nyans');
-    $config->set('giraffe', ['muh', 'hum2']);
-    $config->save();
-
-    $typed_config = $typed_config_manager->get('config_test.validation');
-    $result = $typed_config->validate();
-    $this->assertCount(1, $result);
-    $this->assertEquals('Giraffes just hum', $result->get(0)->getMessage());
-  }
-
-}
diff --git a/core/tests/Drupal/KernelTests/Config/TypedConfigTest.php b/core/tests/Drupal/KernelTests/Config/TypedConfigTest.php
new file mode 100644
index 0000000..4b906b4
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Config/TypedConfigTest.php
@@ -0,0 +1,126 @@
+<?php
+
+namespace Drupal\KernelTests\Config;
+
+use Drupal\Core\TypedData\ComplexDataDefinitionInterface;
+use Drupal\Core\TypedData\ComplexDataInterface;
+use Drupal\Core\TypedData\Type\IntegerInterface;
+use Drupal\Core\TypedData\Type\StringInterface;
+use Drupal\KernelTests\KernelTestBase;
+use Symfony\Component\Validator\ConstraintViolationListInterface;
+
+/**
+ * Tests config validation mechanism.
+ *
+ * @group Config
+ */
+class TypedConfigTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['config_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->installConfig('config_test');
+  }
+
+  /**
+   * Verifies that the Typed Data API is implemented correctly.
+   */
+  public function testTypedDataAPI() {
+    /** @var \Drupal\Core\Config\TypedConfigManagerInterface $typed_config_manager */
+    $typed_config_manager = \Drupal::service('config.typed');
+    /** @var \Drupal\Core\Config\Schema\TypedConfigInterface $typed_config */
+    $typed_config = $typed_config_manager->get('config_test.validation');
+
+    // Test a primitive.
+    $string_data = $typed_config->get('llama');
+    $this->assertInstanceOf(StringInterface::class, $string_data);
+    $this->assertEquals('meh', $string_data->getValue());
+
+    // Test complex data.
+    $mapping = $typed_config->get('cat');
+    /** @var \Drupal\Core\TypedData\ComplexDataInterface $mapping */
+    $this->assertInstanceOf(ComplexDataInterface::class, $mapping);
+    $this->assertInstanceOf(StringInterface::class, $mapping->get('type'));
+    $this->assertEquals('kitten', $mapping->get('type')->getValue());
+    $this->assertInstanceOf(IntegerInterface::class, $mapping->get('count'));
+    $this->assertEquals(2, $mapping->get('count')->getValue());
+    $this->assertInstanceOf(ComplexDataDefinitionInterface::class, $mapping->getDataDefinition());
+    // @todo: Make metadata about contained properties available and add test
+    // coverage here.
+
+    // Test accessing sequences.
+    $sequence = $typed_config->get('giraffe');
+    /** @var \Drupal\Core\TypedData\ListInterface $sequence */
+    $this->assertInstanceOf(ComplexDataInterface::class, $sequence);
+    $this->assertInstanceOf(StringInterface::class, $sequence->get('hum1'));
+    $this->assertEquals('hum1', $sequence->get('hum1')->getValue());
+    $this->assertEquals('hum2', $sequence->get('hum2')->getValue());
+    $this->assertEquals(2, count($sequence->getIterator()));
+    // Verify the item metadata is available.
+    $this->assertInstanceOf(ComplexDataDefinitionInterface::class, $sequence->getDataDefinition());
+    // @todo: Make metadata about contained properties available and add test
+    // coverage here.
+  }
+
+  /**
+   * Tests config validation via the typed data api.
+   */
+  public function testSimpleConfigValidation() {
+    $config = \Drupal::configFactory()->getEditable('config_test.validation');
+    /** @var \Drupal\Core\Config\TypedConfigManagerInterface $typed_config_manager */
+    $typed_config_manager = \Drupal::service('config.typed');
+    /** @var \Drupal\Core\Config\Schema\TypedConfigInterface $typed_config */
+    $typed_config = $typed_config_manager->get('config_test.validation');
+
+    $result = $typed_config->validate();
+    $this->assertInstanceOf(ConstraintViolationListInterface::class, $result);
+    $this->assertEmpty($result);
+
+    // Test constraints on primitive types.
+    $config->set('llama', 'muh');
+    $config->save();
+
+    $typed_config = $typed_config_manager->get('config_test.validation');
+    $result = $typed_config->validate();
+    // Its not a valid llama anymore.
+    $this->assertCount(1, $result);
+    $this->assertEquals('no valid llama', $result->get(0)->getMessage());
+
+    // Test constraints on mapping.
+    $config->set('llama', 'meh');
+    $config->set('cat.type', 'nyans');
+    $config->save();
+
+    $typed_config = $typed_config_manager->get('config_test.validation');
+    $result = $typed_config->validate();
+    $this->assertEmpty($result);
+
+    // Test constrains on nested mapping.
+    $config->set('cat.type', 'miaus');
+    $config->save();
+
+    $typed_config = $typed_config_manager->get('config_test.validation');
+    $result = $typed_config->validate();
+    $this->assertCount(1, $result);
+    $this->assertEquals('no valid cat', $result->get(0)->getMessage());
+
+    // Test constrains on sequences.
+    $config->set('cat.type', 'nyans');
+    $config->set('giraffe', ['muh', 'hum2']);
+    $config->save();
+
+    $typed_config = $typed_config_manager->get('config_test.validation');
+    $result = $typed_config->validate();
+    $this->assertCount(1, $result);
+    $this->assertEquals('Giraffes just hum', $result->get(0)->getMessage());
+  }
+
+}
diff --git a/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php b/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php
index dcc7610..d9a7100 100644
--- a/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php
@@ -65,7 +65,6 @@ function testSchemaMapping() {
     $expected['type'] = 'config_schema_test.someschema';
     $expected['definition_class'] = '\Drupal\Core\TypedData\MapDataDefinition';
     $expected['unwrap_for_canonical_representation'] = TRUE;
-    $this->assertEqual($definition, $expected, 'Retrieved the right metadata for nonexistent configuration.');
     $this->assertEqual($definition, $expected, 'Retrieved the right metadata for configuration with only some schema.');
 
     // Check type detection on elements with undefined types.
@@ -93,7 +92,6 @@ function testSchemaMapping() {
     $expected['type'] = 'undefined';
     $expected['definition_class'] = '\Drupal\Core\TypedData\DataDefinition';
     $expected['unwrap_for_canonical_representation'] = TRUE;
-    $this->assertEqual($definition, $expected, 'Retrieved the right metadata for nonexistent configuration.');
     $this->assertEqual($definition, $expected, 'Automatic type detected for an undefined integer is undefined.');
 
     // Simple case, straight metadata.
@@ -113,7 +111,6 @@ function testSchemaMapping() {
     $expected['type'] = 'system.maintenance';
     $expected['definition_class'] = '\Drupal\Core\TypedData\MapDataDefinition';
     $expected['unwrap_for_canonical_representation'] = TRUE;
-    $this->assertEqual($definition, $expected, 'Retrieved the right metadata for nonexistent configuration.');
     $this->assertEqual($definition, $expected, 'Retrieved the right metadata for system.maintenance');
 
     // Mixed schema with ignore elements.
@@ -145,7 +142,6 @@ function testSchemaMapping() {
     );
     $expected['type'] = 'config_schema_test.ignore';
     $expected['unwrap_for_canonical_representation'] = TRUE;
-    $this->assertEqual($definition, $expected, 'Retrieved the right metadata for nonexistent configuration.');
 
     $this->assertEqual($definition, $expected);
 
@@ -157,7 +153,6 @@ function testSchemaMapping() {
     $expected['class'] = '\Drupal\Core\Config\Schema\Ignore';
     $expected['definition_class'] = '\Drupal\Core\TypedData\DataDefinition';
     $expected['unwrap_for_canonical_representation'] = TRUE;
-    $this->assertEqual($definition, $expected, 'Retrieved the right metadata for nonexistent configuration.');
     $this->assertEqual($definition, $expected);
     $definition = \Drupal::service('config.typed')->get('config_schema_test.ignore')->get('indescribable')->getDataDefinition()->toArray();
     $expected['label'] = 'Indescribable';
@@ -193,7 +188,6 @@ function testSchemaMapping() {
     $expected['mapping']['third_party_settings']['sequence']['type'] = '[%parent.%parent.%type].third_party.[%key]';
     $expected['mapping']['_core']['type'] = '_core_config_info';
     $expected['type'] = 'image.style.*';
-    $this->assertEqual($definition, $expected, 'Retrieved the right metadata for nonexistent configuration.');
 
     $this->assertEqual($definition, $expected);