This chapter examines the fundamental concepts and notions of international criminal law, which is linked to other key areas of international law, particularly human rights, international humanitarian law, immunities, and jurisdiction. In particular, there is a focus on the concept of individual criminal responsibility under international law. The four core crimes are considered; namely, genocide, crimes against humanity, grave breaches of the 1949 Geneva Conventions, and the crime of aggression. Moreover, attention is paid to two unique forms of participation in international crimes, namely, command responsibility and joint criminal enterprise. Finally, the chapter addresses enforcement of international criminal law, particularly through international criminal tribunals, with an emphasis on the International Criminal Court (ICC).