Customary International Law



According to Article 38 of its Statute, the International Court of Justice ‘whose function is to decide in accordance with international law such disputes as are submitted to it,’ has to apply, inter alia, ‘international custom.’ This source of public international law is described, in the same Article, as ‘evidence of a general practice accepted as law.’ This description of international custom, even though it has been criticized for its exact formulation, at least makes clear that international custom generally refers to a description State practice, but only such practice as is accepted by the States themselves as legally required. Once a certain practice is understood to be customary law, States are obliged to act as the rule of customary international law prescribes.

International customary law is probably the most disputed and discussed source of international law. For example, it is not clear when a particular State practice becomes a legally binding State practice. It is also unclear how one can identify a rule of international custom, or how one can prove its existence.

This Research Guide is intended as a starting point for research on Customary International Law. It provides the basic legal materials available in the Peace Palace Library, both in print and electronic format. Handbooks, leading articles, bibliographies, periodicals, serial publications and documents of interest are presented in the Selective Bibliography section. Links to the PPL Catalogue are inserted. The Library’s classification index code 30. Sources and subject heading (keyword) Customary International Law are instrumental for searching through the Catalogue. Special attention is given to our subscriptions on databases, e-journals, e-books and other electronic resources. Finally, this Research Guide features links to relevant websites and other online resources of particular interest.


Reference works


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  • Pedretti, R., Immunity of Heads of State and State Officials for International Crimes, Leiden, Boston, Brill Nijhoff, 2015.

    Immunity of Heads of State and State Officials for International Crimes

    Ramona Pedretti offers, for the first time, a comprehensive assessment of the rules of customary international law relating to immunity of Heads of State and other State officials in the context of crimes pursuant to international law and their relationship with core principles of international law. The book gives the reader a full picture of this topical issue which is located at the heart of today’s development of international law. It contains an in-depth evaluation of a vast amount of relevant material, ranging from domestic laws to judicial decisions of domestic and international courts. The fact that the International Law Commission is deliberating the issue with a view to drafting an international treaty underscores the book’s importance and timeliness.

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  • Bernstein, L., Parisi, F. (eds.), Customary Law and Economics, Cheltenham, UK, Northampton, MA, USA, Edward Elgar, 2014.

    Customary Law And Economics

    Customary law has been the subject of intense debate and the issues arising from the intersection of customs and the law are far from settled. This volume, separated into three parts brings together seminal work from scholars in law, economics and history. The first section analyses various perspectives on the history of customary law. Part two focuses on the commercial customary law and includes a number of case studies covering the role and limits of customary systems in a variety of commercial settings. The final section explores the role of custom in international law from a variety of legal and economic perspectives.

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  • Barrat, C., Status of NGOs in International Humanitarian Law, Leiden, Boston, Brill Nijhoff, 2014.

    Status of NGOs in International Humanitarian Law

    In Status of NGOs in International Humanitarian Law, Claudie Barrat examines the legal framework applicable to NGOs in situations of armed conflict. The author convincingly demonstrates, contrary to convention, that in addition to the ICRC, the National Societies and the IFRC, numerous other NGOs referenced in humanitarian law treaties have a legal status in IHL and therefore legitimate claim to employ IHL provisions to respond to current challenges. On the basis of clear and thorough definitions of these entities, Barrat argues that existing NGOs meeting stringent definition can benefit from customary rights and obligations in both international and non-international armed conflict.

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  • Arajärvi, N., The Changing Nature of Customary International Law: Methods of Interpreting the Concept of Custom in International Criminal Tribunals, London; New York, Routledge, Taylor and Francis Group, 2014.

    Arajärvi, N., The Changing Nature of Customary International Law: Methods of Interpreting the Concept of Custom in International Criminal Tribunals, London; New York, Routledge, Taylor and Francis Group, 2014.

    This book examines the evolution of customary international law (CIL) as a source of international law. Using the International Criminal Tribunal for the former Yugoslavia (ICTY) as a key case study, the book explores the importance of CIL in the development of international criminal law and focuses on the ways in which international criminal tribunals can be said to change the ways in which CIL is formed and identified. In doing so, the book surveys the process and substance of CIL, as well as the problematic distinction between the elements of state practice and opinio juris. By applying an inclusive positivist approach, Noora Arajärvi analyses the methodologies of identification of CIL in selected cases of the ICTY, and their normative foundations. Through examination of the case-law and the reasoning of courts and tribunals, Arajärvi demonstrates to what extent the court’s chosen method of identification of CIL affects the process of custom formation and the resulting system of norms in general. The book will be of great value to researchers and scholars of international law, international relations, and practitioners with interests in customary international law.

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  • Scharf, M., Customary International Law in Times of Fundamental Change: Recognizing Grotian Moments, Cambridge, Cambridge University Press, 2013.

    Customary international law in times of fundamental change : recognizing Grotian moments

    This is the first book to explore the concept of “Grotian Moments.” Named for Hugo Grotius, whose masterpiece De Jure Belli ac Pacis helped marshal in the modern system of international law, Grotian Moments are transformative developments that generate the unique conditions for accelerated formation of customary international law. In periods of fundamental change, whether by technological advances, the commission of new forms of crimes against humanity, or the development of new means of warfare or terrorism, customary international law may form much more rapidly and with less state practice than is normally the case to keep up with the pace of developments. The book examines the historic underpinnings of the Grotian Moment concept, provides a theoretical framework for testing its existence and application, and analyzes six case studies of potential Grotian Moments: Nuremberg, the continental shelf, space law, the Yugoslavia Tribunal’s Tadic decision, the 1999 NATO intervention in Serbia, and the 9/11 terrorist attacks.

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  • Bradley, C., International Law in the U.S. Legal System, New York, NY, Oxford University Press, 2013.

    International law in the U.S. legal system

    The war on terrorism has rapidly increased the importance of international law in the United States. U.S. courts have seen a substantial increase in cases raising issues of international law, and judges have expressed a growing interest in, and appreciation for the topic. This heightened internationalism in the U.S. judiciary has been extremely controversial because of the level of interpretation inherent in the application of international law in our domestic courts. International Law in the U.S. Legal System decodes the often complicated ways that international law operates within the United States legal system and sheds light on unresolved issues and areas of controversy. The book covers all of the principal forms of international law including treaties, decisions and orders of international institutions, customary international law, jus cogens norms, and general principles. It also explores a number of issues that are implicated by the intersection of U.S. law and international law, such as foreign sovereign immunity, international human rights litigation, extradition, and extraterritoriality.

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  • Happy Retirement Ingrid!

    [On the retirement of our curator Ingrid Kost; blog in Dutch] Vandaag 15 januari 2015 is een memorabele dag voor de bibliotheek van het Vredespaleis. Wij nemen na 39 jaar afscheid van onze collega Ingrid Kost. Zij zal genieten van een welverdiend pensioen. Tijd om andere dingen te gaan doen, zoals oppassen op de kleinkinderen en bijenhouden. Wij zullen haar deerlijk missen als collega en als mens. Alvorens vandaag afscheid te nemen, spraken wij met haar en haalden herinneringen op.

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  • Conference about ICJ's judgment in the case between Nicaragua and the USA

    In 1986, the International Court of Justice issued its judgment on the merits in a dispute between Nicaragua and the United States of America. Twenty-five years later, members of the legal teams of both Nicaragua and the United States faced each other once again in the Peace Palace.

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See also

More Research guides on Public International Law

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