diff --git a/docs/installation.rst b/docs/installation.rst
index 868e0e23..d61acd1e 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -114,3 +114,6 @@ Run the development server and visit ``http://localhost:8000``:
 .. code-block:: shell
 
     python manage.py runserver
+
+To be able to extract posters from videos or get the duration of an audio
+file you need to install `ffmpeg <https://ffmpeg.org/download.html>`_.