The purpose of international criminal law is to establish the criminal responsibility of individuals for international crimes. Public international law is traditionally focused on the rights and obligations of states, and thus is not particularly well suited to this task. It has adapted through a long and slow historical process, drawing upon multiple sources. Many of the chapters in this Handbook explore to some extent the historical development of international criminal law. I will not attempt to summarize that history in detail, but a few historical observations here will help to explain how international criminal law emerged from its sources in public international law, comparative law, international humanitarian law and international human rights law. This will set the stage for an introductory discussion of some key issues in contemporary international criminal law.