Customary International Law


Updated: January 8, 2019 (Bibliography).

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 of 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.

The International Law Commission appointed Sir Micheal Wood as Special Rapporteur concerning the issue of Customary International Law. His reports and recommendations are available in the bibliography under documents.

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 systematic classification → Customary international law 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





2016 and before



Systematic classification → Customary international law

New titles

As we are right in the middle of moving to a new library system, it is not yet possible to automatically collect new titles for this Research Guide.

Librarian's choice

  • Rauter, T., Judicial Practice, Customary International Criminal Law and Nullum Crimen Sine Lege, Cham, Springer, 2017.

    Rauter, T., Judicial Practice, Customary International Criminal Law and Nullum Crimen Sine Lege, Cham, Springer, 2017.

    This study analyzes the methods used by international criminal tribunals when determining customary international criminal law and to consider the compatibility of these approaches with the nullum crimen sine lege principle. In this context, the following research questions are of particular importance: Is there one approach common to all international criminal tribunals when determining customary international law? Do international criminal tribunals regard both traditional elements of customary international law - State practice and opinio iuris - as necessary elements for the establishment of customary international law? Do international criminal tribunals argue along the lines of the International Court of Justice (ICJ), requiring a high frequency and consistency of State practice that is both "extensive and virtually uniform"? In addition, the book analyzes the evidence used by international criminal tribunals in order to establish the constituent elements of customary international. It then poses the question: Do international criminal tribunals distinguish, as defined by Schwarzenberger, between the "law-creating processes" of public international law on the one hand, and the "law-determining agencies" as a subsidiary means of determining rule of law on the other? Assuming that they exist, how can different methodological approaches to determine customary international law be assessed in light of the nullum crimen sine lege principle? Does the principle require judges to apply the traditional method to establish customary international law as being based on extensive, uniform and enduring State practice accompanied by opinio iuris? Can the principle balance the desire for justice and the specificities of law creation of the international legal order with fairness for the accused? How can the law be accessible and criminal punishment foreseeable, when the underlying legal basis for criminal convictions, namely customary international criminal law, is unwritten in nature?

    View this title in our discovery service
  • Lepard, B.D. (ed.), Reexamining Customary International Law, Cambridge, Cambridge University Press, 2017.

    Lepard, B.D. (ed.), Reexamining Customary International Law, Cambridge, Cambridge University Press, 2017.

    Tis book takes on the complex issues and controversies surrounding the history, theory, and practice of customary international law as it reexamines customary law's increasingly important role in world affairs. It incorporates the expertise of distinguished authors to probe many difficult issues that remain unresolved concerning the doctrine of customary law. At the same time, this book engages in a profound exploration of the practical role of customary international law in a variety of important fields, including humanitarian law, human rights law, and air and space law.

    View this title in our discovery service
  • Bradley, C.A., Custom's Future: International Law in a Changing World, New York, NY, Cambridge University Press, 2016.

    Although customary international law has long been an important source of rights and obligations in international relations, there has been extensive debate in recent years about whether this body of law is equipped to address complex modern problems such as climate change, international terrorism, and global financial instability. In addition, there is growing uncertainty about how, precisely, international and domestic courts should identify rules of customary international law. Custom's Future seeks to address this uncertainty by providing a better understanding of how customary international law has developed over time, the way in which it is applied in practice, and the challenges that it faces going forward. Reflecting an interdisciplinary mix of historical, empirical, economic, philosophical, and doctrinal analysis, and containing chapters by leading international law experts, it will be of use to lawyers, judges, and researchers alike.

    View this title in our discovery service
  • Council of Europe, The Judge and International Custom = Le Juge et la Coutume Internationale, [Strasbourg], [Council of Europe], 2016.

    "Loin d’être archaïque, comme certains pourraient le prétendre, la coutume internationale nous a paru être un sujet de réflexion d’un intérêt particulier dans le cadre d’une conférence intitulée "Le juge et la coutume internationale", qui est venue clore d’une belle façon la 44ème réunion du Comité des conseillers juridiques sur le droit international public (CAHDI) à Paris.

    View this title in our discovery service
  • Green, J.A., The Persistent Objector Rule in International Law, Oxford, United Kingdom, Oxford University Press, 2016.

    The persistent objector rule is said to provide states with an 'escape hatch' from the otherwise universal binding force of customary international law. It provides that if a state persistently objects to a newly emerging norm of customary international law during the formation of that norm, then the objecting state is exempt from the norm once it crystallizes into law. The conceptual role of the rule may be interpreted as straightforward: to preserve the fundamentalist positivist notion that any norm of international law can only bind a state that has consented to be bound by it. In reality, however, numerous unanswered questions exist about the way that it works in practice. Through focused analysis of state practice, this monograph provides a detailed understanding of how the rule emerged and operates, how it should be conceptualized, and what its implications are for the binding nature of customary international law. It argues that the persistent objector rule ultimately has an important role to play in the mixture of consent and consensus that underpins international law.


    View this title in our discovery service
  • Dumberry,P., The Formation and Identification of Rules of Customary International Law in International Investment Law, Cambridge, United Kingdom, Cambridge University Press, 2016.


    Rules of customary international law provide basic legal protections to foreign investors doing business abroad. These rules remain of fundamental importance today despite the growing number of investment treaties containing substantive investment protection. In this book, Patrick Dumberry provides a comprehensive analysis of the phenomenon of custom in the field of international investment law. He analyses two fundamental questions: how customary rules are created in this field and how they can be identified. The book examines the types of manifestation of State practice which should be considered as relevant evidence for the formation of customary rules, and to what extent they are different from those existing under general international law. The book also analyses the concept of States' opinio juris in investment arbitration. Offering guidance to actors called upon to apply customary rules in concrete cases, this book will be of significant importance to those involved in investment arbitration.

    View this title in our discovery service
  • Kolb, R., Peremptory International Law - Jus Cogens: a General Inventory, Oxford, Hart Publishing, Bloomsbury Publishing plc, 2015.


    Given the literature in the field of jus cogens one might ask what could possibly be added to the body of literature already existing in field? Robert Kolb, one of the leading international scholars of his generation, offers a seminal survey of the question of peremptory international law. The book analyses and systemises courts’ diverging approaches, and draws a typology of techniques for judicial protection afforded to individuals affected by UNSC measures. On the basis of this analysis, the book identifies the discrepancies with the international human rights law standards and proposes solutions. The study pays special attention to the persisting problem with the targeted sanctions regime, which is that the grounds and evidence on the basis of which individuals are designated remain largely confidential. The book suggests an amendment to the present UNSC procedure, which would mitigate this problem. This important book is essential reading for all scholars of the subject.

    View this title in our discovery service



  • 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.

    Read more
  • 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.

    Read more

See also

More Research guides on Public International Law

Other suggestions

Systematic classification → Customary international law