This chapter explores the treatment of the principle of legality in international criminal law, in particular the rule against ex post facto application of criminal laws, as enshrined in human rights law. It demonstrates that a broadly liberal interpretation of nullum crimen has facilitated judicial creativity and the development of international criminal law by international courts and tribunals. The chapter begins with a general discussion of the principle of legality under international law, before turning to a consideration of the treatment of the principle at Nuremberg and the ad hoc international criminal tribunals for Rwanda and the former Yugoslavia. The final section of the chapter turns to the European Court of Human Rights and examines how it has addressed the rule of non-retroactivity in the context of national prosecutions of international crimes, in particular in Kononov v. Latvia.