The aim of this contribution on G.A. van Hamel, one of the co-founders of the I.K.V. is double. First, what were the international schools of thought that inspired him, as a pupil of the neo-classic school, to become a leading figure of penal positivism and social defence and what was his own impact in this international arena? Second, to which extent was he able to elaborate, in his scholarly and policy work, a proper criminal justice theory of social defence?