"This edited volume finds its origin in a conference on the theory and practice of customary international law (CIL) and its interpretation, held in May 2019 at the University of Groningen. The conference was co-organized by the TRICI-Law project and the Interest Group on International Legal Theory and Philosophy (IGILTP)."--ECIP Preface.

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.