Titel: Isometric CAT(0) actions and centralizers in Aut(F_n) abstract: The main theorem of my talk is that Nielsen generators always act by zero translation length whenever Aut(F_n) acts by isometries on a complete CAT(0) space and n is at least 4. Results like this are interesting to study group homomorphisms out of Aut(F_n). The first proof of the theorem uses a parallelogram law for the translation lengths of two commuting isometries on a CAT(0) space. The second proof requires centralizers in Aut(F_n). Finally I will sketch how to obtain a finite presentation for some centralizers in Aut(F_n).