diff --git a/src/Annotation/FacetsHierarchy.php b/src/Annotation/FacetsHierarchy.php
index d5f82cd..d3ec151 100644
--- a/src/Annotation/FacetsHierarchy.php
+++ b/src/Annotation/FacetsHierarchy.php
@@ -7,7 +7,7 @@ use Drupal\Component\Annotation\Plugin;
 /**
  * Defines a Facets Hierarchy annotation.
  *
- * @see \Drupal\facets\Processor\ProcessorPluginManager
+ * @see \Drupal\facets\Hierarchy\HierarchyPluginManager
  * @see plugin_api
  *
  * @ingroup plugin_api
diff --git a/src/FacetManager/DefaultFacetManager.php b/src/FacetManager/DefaultFacetManager.php
index 6abc8ae..b623d37 100644
--- a/src/FacetManager/DefaultFacetManager.php
+++ b/src/FacetManager/DefaultFacetManager.php
@@ -389,6 +389,21 @@ class DefaultFacetManager {
     return !empty($this->facets[$facet->id()]) ? $this->facets[$facet->id()] : NULL;
   }
 
+  /**
+   * Builds an hierarchical structure for results.
+   *
+   * When given an array of results and an array which defines the hierarchical
+   * structure, this will build the results structure and set all childs.
+   *
+   * @param \Drupal\facets\Result\ResultInterface[]
+   *   An array of results keyed by id.
+   *
+   * @param array
+   *   An array of 'child id arrays' keyed by their parent id.
+   *
+   * @return \Drupal\facets\Result\ResultInterface[]
+   *   An array of results structured hierarchicaly.
+   */
   protected function buildHierarchicalTree($keyed_results, $parent_groups) {
     foreach ($keyed_results as &$result) {
       $current_id = $result->getRawValue();
diff --git a/src/Hierarchy/HierarchyInterface.php b/src/Hierarchy/HierarchyInterface.php
index b67147b..b7c367c 100644
--- a/src/Hierarchy/HierarchyInterface.php
+++ b/src/Hierarchy/HierarchyInterface.php
@@ -5,8 +5,26 @@ namespace Drupal\facets\Hierarchy;
 
 interface HierarchyInterface {
 
+ /**
+  * Retrieve all parent ids for one specific id.
+  *
+  * @param string $id
+  *   An entity id.
+  *
+  * @return array
+  *   An array of all parent ids.
+  */
   public function getParentIds($id);
 
+ /**
+  * Retrieve all children and nested children for one specific id.
+  *
+  * @param string $id
+  *   An entity id.
+  *
+  * @return array
+  *   An array of all child ids.
+  */
   public function getNestedChildIds($id);
 
  /**