This book examines the evolution of customary international law (CIL) as a source of international law. Using the International Criminal Tribunal for the Former Yugoslavia (ICTY) as a key case study, the book explores the importance of CIL in the development of international criminal law and focuses on the ways in which international criminal courts and "hybrid" criminal tribunals can be said to change the ways in which CIL is determined. In doing so, the book surveys the process and substance of CIL, as well as the problematic distinction between the elements of state practice and opinio juris.