Customary international law remains a central source of international law and the core of the international legal system. It continues to draw the attention of lawyers, especially at a time marked by the great expansion of international law and its increasing application in domestic and international courts. Determining whether an applicable rule of customary international law exists is therefore of great practical concern - but this important legal task is not always simple or straightforward.

This book serves as guidance to those seeking to determine the existence of rules of customary international law and their content. It elaborates on the methodology for the identification of rules of customary international law and examines a host of questions concerning the process and evidence at issue. It does so by complementing the authoritative work of the UN International Law Commission on this topic, and by drawing upon a wealth of additional practice and writings.

Identification of Customary International Law provides an overview of the Commission's work and expands on it by addressing the nature and history of custom as a source of international law, inquiring into each of the two constituent elements of customary international law (namely, a general practice and opinio juris), explaining the value and limits of certain forms of evidence, and throwing further light on such issues as the persistent objector rule and particular customary international law. Practitioners and scholars alike will find this detailed treatment useful in seeking to determine the existence and content of any customary rule and in ensuring that arguments about customary international law are persuasive.

Michael Wood, Barrister, 20 Essex Street Chambers,Omri Sender, Partner and Chair of the Public International Law Practice Group, S. Horowitz & Co.

Sir Michael Wood is a barrister at Twenty Essex, London, where he practises in the field of public international law, including before international courts and tribunals. He was Legal Adviser to the United Kingdom's Foreign and Commonwealth Office between 1999 and 2006, having joined as an Assistant Legal Adviser in 1970. He was a member of the UN International Law Commission from 2008 to 2022, and its Special Rapporteur on the topic 'Identification of customary international law'. He is an Honorary Fellow of the Lauterpacht Centre for International Law, University of Cambridge.


Dr Omri Sender is Partner and Chair of the Public International Law Practice Group at S. Horowitz & Co, acting as counsel and advisor to States, international organizations, and multinational corporations on issues covering the full range of public international law. He has particular experience representing and advising States in international dispute settlement proceedings, including before the International Court of Justice. He previously served as Counsel at the World Bank, as Legal Assistant at the United Nations International Law Commission, and as Law Clerk to the Chief Justice of the Supreme Court of Israel.

1. Introduction
2. Custom as a Source of International Law
3. The International Law Commission's Work on Identification of Customary International Law
4. The Two-element Approach
5. Assessment of Evidence for the Two Constituent Elements
6. A General Practice: Whose Practice? What Practice?
7. A General Practice: Assessing Generality
8. Acceptance as Law (Opinio Juris)
9. The Significance of Treaties for the Identification of Customary International Law
10. Resolutions of International Organizations and Intergovernmental Conferences as Evidence of Customary International Law
11. Judicial Decisions and the Teachings of Publicists as Subsidiary Means for the Determination of Rules of Customary International Law
12. The Persistent Objector Rule
13. Particular Customary International Law
14. Concluding Observations