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.

Introduction 1. Genesis of Customary International Law and International Criminal Law 2. New Concept of Customary International Law - The Role of International Criminal Judge 3. Customary International Law in the Decisions of the ICTY 4. Principle of Legality and Customary International Law 5. The Need for a New Conceptual Framework for the Sources in International Law General Conclusion

Noora Arajärvi serves in the United Nations as part of the Rule of Law Unit at the EOSG. She has previously worked as a lecturer at UCLan Cyprus, UWI in Trinidad and Tobago, and as a research assistant at Tilburg University in the Netherlands. Dr Arajärvi gained her Ph.D. from the EUI in Florence in 2011. In her recent research, she concentrates on (inclusive) positivist theory of sources in international law.