The legal philosophy of the xx century has contributed to the development of international criminal law by rethinking fundamental legal concepts and theories concerning the nature of international law, its relation with national laws, the connection between the law and the State, and the very idea of responsibility. This was achieved, in the first place, through the reflection of Hans Kelsen, who put forward the idea of a system of enforceable criminal norms at the international level, directed at individuals and having a positive legal foundation. In the years immediately following the Second World War, a number of legal theorists and, in particular, Gustav Radbruch, argued in favour of a necessary connection between law and morality, on whose basis it could be claimed that the worst atrocities were punishable even when allowed by state norms, and even in the absence of positive international norms. In the last decade, the practice of international criminal law, through ad hoc tribunals and the International Criminal Court, has stimulated theoretical reflections on a variety of further fundamental issues, like impartiality, judicial truth, justification of punishment, side-effects of prosecution and transitional justice.