This chapter sets down the legal and historical foundations of international criminal law. It begins with a brief overview of the history of international criminal law, beginning with the 1919 Versailles Peace Treaty and ending with the developments after the creation of the International Criminal Court (ICC) Statute. From there, the chapter discusses the concepts, aim, and legitimacy of international criminal law before turning to the role of international criminal law within the international legal order. Afterward, the chapter turns to the sources and interpretation of international criminal law as well as universal jurisdiction, the duty to prosecute, and transitional justice. Next, the chapter considers the relationship between international and domestic courts as well as the prosecution of international crimes under international law by international and ‘internationalised’ courts. Finally, this chapter closes with a discussion on international criminal law in practice.