 He was a visiting professor at the University of California in Santa Barbara (1967) and Berkeley (1975), the University of Tromsø (1980), also the University of Warsaw (1988) and the University of Bremen (1988).Together with Nils Christie and Louk Hulsman he is a distinguished representative of the prison abolition movement.He writes in Norwegian and English, several of his books have been translated into other languages, including Swedish, Danish, German, Italian and Spanish.