This book evaluates the contribution of contemporary international criminal courts to the evolution of customary international law. It provides critical insight to the progressive development of law by elaborating on the interplay between customary law and judicial decisions and the application of the principle of legality in international criminal proceedings. Focusing on the case law of the modern ad hoc tribunals, this book critically analyzes the discovery and identification of rules of customary international law and asks whether the requirements of the principle of legality have been fulfilled. Finally, it evaluates how the judicial creativity practiced in the modern international criminal tribunals has affected —and how the future work of the ICC will affect— the reception of the principle of legality on the international stage. This book is useful for people interested in international criminal law, its sources and the application of the principle of legality in international law.