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.

The book succeeds in providing a clear, focused account of a highly technical area of international law, effectively synthesising an impressive volume of practice and scholarship, while adding more detailed analysis of issues not covered by, or only touched on in, the ILC's work. ..the authors have succeeded in producing that rare thing: an accomplished academic work that is sufficiently clear and accessible to be read by students studying the sources of international law for the first time and which will be a useful text for practitioners, but which also includes analysis of more complex debates within the subject, and detailed references to practice and literature, such as to make it a valuable research resource for scholars.

• 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

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.

The book succeeds in providing a clear, focused account of a highly technical area of international law, effectively synthesising an impressive volume of practice and scholarship, while adding more detailed analysis of issues not covered by, or only touched on in, the ILC's work. ..the authors have succeeded in producing that rare thing: an accomplished academic work that is sufficiently clear and accessible to be read by students studying the sources of international law for the first time and which will be a useful text for practitioners, but which also includes analysis of more complex debates within the subject, and detailed references to practice and literature, such as to make it a valuable research resource for scholars.

• 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