diff --git a/src/ol/proj/proj.js b/src/ol/proj/proj.js
index 6f8d5112be..2a10cadc37 100644
--- a/src/ol/proj/proj.js
+++ b/src/ol/proj/proj.js
@@ -61,6 +61,19 @@ ol.proj.METERS_PER_UNIT[ol.proj.Units.METERS] = 1;
  * You can use {@link ol.proj.get} to retrieve the object for a particular
  * projection.
  *
+ * The library includes definitions for `EPSG:4326` and `EPSG:3857`, together
+ * with the following aliases:
+ * * `EPSG:4326`: CRS:84, urn:ogc:def:crs:EPSG:6.6:4326,
+ *     urn:ogc:def:crs:OGC:1.3:CRS84, urn:ogc:def:crs:OGC:2:84,
+ *     http://www.opengis.net/gml/srs/epsg.xml#4326,
+ *     urn:x-ogc:def:crs:EPSG:4326
+ * * `EPSG:3857`: EPSG:102100, EPSG:102113, EPSG:900913,
+ *     urn:ogc:def:crs:EPSG:6.18:3:3857,
+ *     http://www.opengis.net/gml/srs/epsg.xml#3857
+ *
+ * If you use proj4js, aliases can be added using `proj4.defs()`; see
+ * [documentation](https://github.com/proj4js/proj4js).
+ *
  * @constructor
  * @param {olx.ProjectionOptions} options Projection options.
  * @struct