Jus cogens is a formidable yet elusive concept of international law. Since its incorporation in the Vienna Convention on the Law of Treaties some 35 years ago, it has made tentative inroads into international legal practice. But its role in international law is arguably less prominent than might have been expected on the basis of its powerful potential and in view of wider developments in international law that call for constitutionalisation and hierarchy, including the processes of fragmentation and humanization. This volume of the Netherlands Yearbook of International Law sets out to clarify the concepts and doctrines relevant to jus cogens and to sharpen the debate on its theoretical foundations, functions and legal effects. To that purpose, the volume brings together contributions on the genesis and function of jus cogens, on the application of jus cogens in specialised areas of international law and on its enforcement and legal consequences.Together, they reinforce the understanding of jus cogens as a hierarchical concept of international law and shed light on its potential for further development.

Jus Cogens and the Humanisation and Fragmentation of International Law.- Sherlock Holmes and the Mystery of Jus Cogens.- Understanding the Jus Cogens Debate: The Pervasive Influence of Legal Positivism and Legal Idealism.- Jus Cogens as a Social Construct without Pedigree: 'If Judges Say so Then it Must be True'.- Audience and Authority - The Merit of the Doctrine of Jus Cogens.- Genesis, Function and Identification of Jus Cogens Norms.- Jus Cogens as the 'Highest Law'? Peremptory Norms and Legal Hierarchies.- In Quest of the Practical Value of Jus Cogens Norms.- Constitutional Conversations in the Anthropocene: In Search of Environmental Jus Cogens Norms.- Non-Refoulement as Custom and Jus Cogens? Putting the Prohibition to the Test.- Improving Compliance: Jus Cogens and International Economic Law.- Jus Cogens in International Investment Law and Arbitration.- Immunities of International Organizations before Domestic Courts: Reflections on the Collective Labour Case against the European Patent Organization.- Judicial Review on the Island of Saint Martin: An Example for the Kingdom of The Netherlands?.- Between Pretence and Practice: The Dutch Response to Recommendations of International Human Rights Bodies.