In this thesis, we develop three lines of investigation around actions and commutators in semi-abelian categories. In the first chapter, we lay the foundations of a theory of commutators twisted by an internal action and apply it to a characterisation of internal crossed modules. The second chapter introduces an intrinsic notion of tensor product—the bilinear product, a cosmash product computed in the two-nilpotent core—and uses it to prove a categorical version of Ganea’s theorem in homology. Finally, in the last chapter, we describe a categorical procedure towards a universal Kaluzhnin–Krasner embedding theorem and carried it out for groups and for precrossed modules.