diff --git a/externs/olx.js b/externs/olx.js
index cbba3e2ae3..9700718064 100644
--- a/externs/olx.js
+++ b/externs/olx.js
@@ -226,7 +226,10 @@ olx.MapOptions.prototype.keyboardEventTarget;
 
 
 /**
- * Layers. If this is not defined, a map with no layers will be rendered.
+ * Layers. If this is not defined, a map with no layers will be rendered. Note
+ * that layers are rendered in the order supplied, so if you want, for example,
+ * a vector layer to appear on top of a tile layer, it must come after the tile
+ * layer.
  * @type {Array.<ol.layer.Base>|ol.Collection.<ol.layer.Base>|undefined}
  * @api stable
  */
diff --git a/src/ol/map.js b/src/ol/map.js
index ebd33d7a5e..1a3a47a991 100644
--- a/src/ol/map.js
+++ b/src/ol/map.js
@@ -151,6 +151,15 @@ ol.MapProperty = {
  * a further element within the viewport, either DOM or Canvas, depending on the
  * renderer.
  *
+ * Layers are stored as a `ol.Collection` in layerGroups. A top-level group is
+ * provided by the library. This is what is accessed by `getLayerGroup` and
+ * `setLayerGroup`. Layers entered in the options are added to this group, and
+ * `addLayer` and `removeLayer` change the layer collection in the group.
+ * `getLayers` is a convenience function for `getLayerGroup().getLayers()`.
+ * Note that `ol.layer.Group` is a subclass of `ol.layer.Base`, so layers
+ * entered in the options or added with `addLayer` can be groups, which can
+ * contain further groups, and so on.
+ *
  * @constructor
  * @extends {ol.Object}
  * @param {olx.MapOptions} options Map options.