The 1969 and 1986 Vienna Conventions on the Law of Treaties are essential components of the contemporary international legal order. Codifying to a significant extent the customary rules that pre-existed in the field, they aim at regulating what has become the main source of public international law and a crucial tool in inter-state relations. This fully updated second edition provides both international and national lawyers with an in-depth analysis of each provision of both Conventions, taking into account the substantial developments of the last decade. The structure of each commentary is essentially uniform: the first part is dedicated to the exposition of that provision's object and purpose and to the assessment of its customary status, while the second part focuses on the main issues of interpretation raised by the provision in question. Extensive reference is made to the travaux préparatoires of both Conventions, including the work of the UN International Law Commission and the proceedings of the 1969 and 1986 diplomatic conferences, and to practice both prior to and following the adoption of the Conventions. The volume contains contributions from over 120 authors from over thirty different countries, including some of the most renowned experts in international law. Not only does this ensure the high quality of the analysis, but it also provides the reader with a reflection of the wide range of perspectives existing in the international scholarship on the law of treaties. "A major strength of the work lies in its systematic examination of each provision's object and purpose, closely reflecting the interpretative approach of international courts and tribunals. Equally valuable is its consistent analysis of the customary status of each article. This allows practitioners to determine applicable rules when dealing with non-parties to the VCLT and mixed treaty regimes, such as those involving the EU." -- Simone Mamini, The Law Society Gazette Olivier Corten, Professor of Public International Law and Director of the Centre de Droit International, Université libre de Bruxelles ,Pierre Klein, Professor of Public International Law and Dean of the Faculty of Law and Criminology, Université libre de Bruxelles ,Vaios Koutroulis, Professor of Public International Law, Université libre de Bruxelles ,Anne Lagerwall, Professor of Public International Law, Université libre de Bruxelles Olivier Corten is a Professor at the Université libre de Bruxelles and Director of the Centre de Droit International. He has taught at various universities in Belgium, Germany, Spain, Portugal, France, Italy, Brazil and Japan, as well as at the UN and The Hague Academy of International Law. He has authored or co-authored 15 books and over 170 articles published in various international law journals. Since 2002, he has been Director, then Co-Director, of the Belgian Review of International Law . In 2019, he was elected Associate Member of the Institut de Droit International, becoming a full Member in 2023. As a lawyer, he has appeared before the International Court of Justice in several cases. Pierre Klein teaches public international law and is Dean of the Faculty of Law and Criminology at the Université libre de Bruxelles. He taught as a Visiting Lecturer or Professor at the Université catholique de Louvain, the University of Ottawa, McGill University, the University of Abomey-Calavi (Bénin), the University of Paris XI, and the Hague Academy of International Law. He is the author or co-author of four books including Bowett's Law of International Institutions (5th ed., Sweet and Maxwell) and more than 70 articles, notes, and chapters published in various international law journals and books. He is also active as a consultant to various States and international organizations, and has acted as counsel in numerous cases before the International Court of Justice and the International Tribunal for the Law of the Sea. Vaios Koutroulis is a Professor at the Université libre de Bruxelles. He has given numerous lectures both in Belgium and abroad, including lectures on international humanitarian law and international criminal law for members of the armed forces, humanitarians, and other professionals. Since 2022, he has been a Member of the International Humanitarian Fact-Finding Commission. He is a General Editor of The Military Law and the Law of War Review and a member of the editorial board of the Belgian Review of International Law . As a practitioner, he was a member of the legal team of Belgium before the International Court of Justice in 2012 and 2024. He has advised the Belgian Ministry of Foreign Affairs on international criminal law questions relating to the Ljubljana-The Hague Convention and chaired one of the three working groups at the negotiations of the convention. Anne Lagerwall is a Professor at the Université libre de Bruxelles. She has given lectures in universities in Belgium, France, The Netherla