Panos Merkouris is Professor of International Law at the University of Groningen. He holds a Chair on Interpretation & Dispute Settlement in International Law. He is the Principal Investigator of the TRICI-Law project. Professor Merkouris has written extensively on interpretation, most recently co-authoring Treaties in Motion (2020) with Professor Malgosia Fitzmaurice. Jörg Kammerhofer is Senior Research Fellow at the University of Freiburg, Germany, and Privatdozent for international law and legal theory at the Vienna University of Economics. He is a generalist international law scholar, specialising in legal theory, and has published widely, including, recently, International Investment Law and Legal Theory: Expropriation and the Fragmentation of Sources (2021). Noora Arajärvi is Postdoctoral Fellow with the ERC funded project 'Cultural Expertise in Europe: What is it useful for?', and Associate at the Hertie School. She is the author of The Changing Nature of Customary International Law (2014) and several journal articles and book chapters on customary international law and other key questions of international law.

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• Part I. The Theory of Customary International Law: Fault Lines and the Need for New Approaches: 1. Between pragmatism and disenchantment: The theory of customary international law after the ILC Project Jörg Kammerhofer; 2. The custom-making moment in customary international law Jean d'Aspremont; 3. Misinterpreting customary international law: Corrupt pedigree or self-fulfilling prophecy? Noora Arajärvi; 4. The logic of absence in customary international law: An open-system approach Anna Irene Baka; 5. Schrödinger's custom: Implications of identification on the interpretation of customary international law Markus P. Beham; Part II. Customary International Law as a Source of International Law: Doctrine and History: 6. The significance of state consent for the legitimate authority of customary international law Andreas Føllesdal; 7. Custom and the regulation of 'the sources of international law' Diego Mejía-Lemos; 8. The ILC's new way of codifying international law, the motives behind it, and the interpretive approach best suited to it Luigi Crema; 9. Beyond formalism: Reviving the legacy of Sir Henry Maine for customary international law Andreas Hadjigeorgiou; 10. Enkapsis and the development of customary international law: An encyclopedic approach to inter-legality Romel Regalado Bagares; Part III. The Practice of Customary International Law Across Various Fora: Diversity of Approaches and Actors: 11. Customary international law in the reasoning of international courts and tribunals Vladyslav Lanovoy; 12. Eureka! On courts' discretion in 'ascertaining' rules of customary international law Letizia Lo Giacco; 13. Identification of and resort to customary international law by the WTO Appellate Body Mariana Clara De Andrade; 14. The practice of non-state armed groups and the formation of customary international humanitarian law: Towards direct relevance? Zhuo Liang; 15. Identifying custom in universal periodic review recommendations Frederick Cowell; Part IV. Interpretation of Customary International Law: Delineating the Stages in its Life Cycle: 16. Interpreting customary international law: you'll never walk alone Panos Merkouris; 17. Practical reasoning and interpretation of customary international law Kostiantyn Gorobets; 18. Different strings of the same harp: interpretation of rules of customary international law, their identification and treaty interpretation Marina Fortuna; 19. Customary international law: identification versus interpretation Riccardo Di Marco; 20. 'And in the darkness bind them': hand-waving, bootstrapping, and the interpretation of customary international law after Chagos John R. Morss and Emily Forbes; Part V. Customary International Law in the Practice of Domestic Courts: What Lessons for International Law?: 21. The role of domestic courts in the interpretation of customary international law: how can we learn from domestic interpretive practices? Nina Mileva; 22. Customary international law interpretation: the role of domestic courts Cedric Ryngaert; 23. The relevance of customary international law in the domestic legal order of a federal state Gerhard Hoogers.