The Theory, Practice, and Interpretation of Customary International Law

Although customary international law (CIL) has been central to international law from its inception, it is often misunderstood. This edited volume remedies that problem by tracing the history of CIL, and providing an in-depth study of its theory, practice, and interpretation. Its chapters tackle the big questions which surround this source of international law such as: what are the rules that regulate the functioning of CIL as a source of international law? Can CIL be interpreted? Where do lines between identification, interpretation, application, and modification of a rule of CIL lie? Using recent developments, this volume revisits old debates and resolves them by proffering new and innovative solutions. With detailed examples from international and national courts, it places CIL in a range of settings to explain, explore and reflect upon this developing and highly significant field. This title is also available as Open Access on Cambridge Core.

Panos Merkouris is Professor at the University of Groningen. He holds a Chair on Interpretation and 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 (Cambridge, 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 (Cambridge, 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.