This chapter deals with European criminal law and its relation to domestic law, international law, transnational law, and criminal justice. It begins with an introduction to general institutional developments in the structures of the European Union that resulted in the emergence of a fragmentary transnational criminal law. In particular, it traces the development of European criminal law to deal with crime control involving terrorism and other crimes of an international character, from the Amsterdam Treaty (1997/1999) to the Framework Decision. It also discusses cooperation in the field of criminal law among the Member States of the European Union. Attention then turns to the general traits of international criminal law. In the EU setting, the adoption of the mutual recognition principle as the cornerstone of criminal law cooperation is given consideration.