Master degree course "Operads & Grothendieck-Teichmüller groups". Twelveth lecture: Recollections and complements on the explicit definition of the Grothendieck-Teichmüller group. §1. Classifying spaces of categories and of operads in groupoids. §2. Homotopy automorphisms of operads. Theorem: The Grothendieck-Teichmüller group is the group of homotopy automorphisms of the little 2-disc operad over Q.