This thesis explores the defence of mistake of law in the context of international criminal proceedings, especially at the International Criminal Court (ICC). In so doing, it first defines the contours of the defence of mistake of law in general theory of criminal law, distinguishing it from cognate defences and highlighting its connection with superior orders. Secondly, the thesis gives an overview of the possible normative approaches to the defence, offering examples from national law as terms of reference for the subsequent analysis of international criminal law. Thirdly, it surveys the relevant law and practice of international criminal tribunals, with a focus on the ICC, and it contemplates offences for which a defence of mistake of law may actually succeed. Fourthly, this thesis draws on the previous analysis, making some observations about the source of international criminal tribunals' legitimacy and about the purpose of punishing individuals at the international level.