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 when such practice is accepted by the States themselves as legally required, it is considered customary law. 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. Important elements are the manifestations of State practice, like declarations, statements or other official documents.
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 in the field of Customary International Law. It provides the basic 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 subject heading (keyword) International Customary Law is 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.
Sources of international law
Treaties
Case-law
UN Declarations and Resolutions
The resolutions and decisions of the United Nations General Assembly and Security Council may provide valuable information. Although they cannot themselves constitute international law or serve as conclusive evidence of a rule of customary law, such resolutions do have value in providing evidence of existing or emerging law. This point of view has been acknowledged by the International Law Commission.
As every Member State has been recognized and has a vote in the UN General Assembly, finding General Assembly resolutions that passed unanimously or near-unanimously may serve as an excellent starting point in the legal research process. Pay attention to the language of the resolution. Those with firm obligations versus those that are merely aspirational or advisory in nature are more likely to be considered valuable as evidence of customary international law.
→ Official Document System Search. ODS is the UN's Official Document System. You can search for UN declarations, resolutions and other documents by keywords, then narrow your search.
→ Start Your Research: (7) Resolutions and Decisions of International Organizations.
Soft law
- International Law Association, Final Report of the Committee on Formation of Customary (General) International Law, London Conference (2000).
- International Law Commission, Formation and Evidence of Customary International Law, Note by M. Wood, Special Rapporteur, UN Doc. A/CN.4/653 (2012).
- International Law Commission, First Report on Formation and Evidence of Customary International Law, by M. Wood, Special Rapporteur, UN Doc. A/CN.4/663 (2013).
- International Law Commission, Second Report on Identification of Customary International Law, by M. Wood, Special Rapporteur, UN Doc. A/CN.4/672 (2014).
- International Law Commission, Third Report on Identification of Customary International Law, by M. Wood, Special Rapporteur, UN Doc. A/CN.4/682 (2015).
- International Law Commission, Fourth Report on Identification of Customary International Law, by M. Wood, Special Rapporteur, UN Doc. A/CN.4/695 (2016).
- International Law Commission, Fifth Report on Identification of Customary International Law, by M. Wood, Special Rapporteur, UN Doc. A/CN.4/717 (2018).
- International Law Commission, The Role of Decisions of National Courts in the Case-law of International Courts and Tribunals of a Universal Character for the Purpose of the Determination of Customary International Law, Memorandum by the Secretariat, UN Doc. A/CN.4/691 (2016).
- International Law Commission, Report on the Work of the Sixty-Eighth Session, Chapter. V: Identification of Customary International Law (draft conclusions), UN Doc. A/71/10 (2016).
- International Law Commission, Report on the Work of the Seventieth Session, Chapter. V: Identification of Customary International Law (draft conclusions with commentaries), UN Doc. A/73/10 (2018).
Reference works
- Bederman, D.J., Custom as a Source of Law, New York, NY, Cambridge University Press, 2010.
- D'Aspremont, J., The Discourse on Customary International Law, Oxford, Oxford University Press, 2021.
- Dupuy, P.-M. (ed.), Customary International Law, Cheltenham; Northampton, Edward Elgar Publishing, 2021.
- Lepard, B.D., Customary International Law: A New Theory with Practical Applications, Cambridge, Cambridge University Press, 2010.
- Mendelson, M.H. “The Formation of Customary International Law”, Recueil des cours=Collected Courses of the Hague Academy of International Law, 272 (1998), pp. 155–410.
- Shaw, M.N., International Law, 7th ed., Cambridge, Cambridge University Press, 2014.
- Wolfke, K., Custom in Present International Law (2nd ed.), Dordrecht, Nijhoff, 1993.
Selected books and articles
- D'Amato, A., The Concept of Custom in International Law, Ithaca, NY, Cornell University Press, 1971.
- Arajärvi, N., The Changing Nature of Customary International Law: Methods of Interpreting the Concept of Custom in International Criminal Tribunals, London, Routledge, Taylor and Francis Group, 2014.
- Bradley, C.A. (ed.), Custom's Future: International Law in a Changing World, New York, NY, Cambridge University Press, 2016. [e-book]
- Byers, M., Custom, Power and the Power of Rules: International Relations and Customary International Law, Cambridge, Cambridge University Press, 1999.
- Cannizzaro, E., and P. Palchetti (eds.), Customary International Law on the Use of Force : A Methodological Approach, Leiden, Nijhoff, 2005.
- Dumberry, P., The Formation and Identification of Rules of Customary International Law in International Investment Law, Cambridge, Cambridge University Press, 2016.
- http://www.ppl.nl/link.php?num=259166901.
- Lepard, B.D. (ed.), Reexamining Customary International Law, Cambridge, Cambridge University Press, 2017.
- Lijnzaad, L., and Council of Europe (eds.), The Judge and International Custom = Le juge et la coutume internationale, Leiden, Brill Nijhoff, 2016.
- Rauter, T., Judicial Practice, Customary International Criminal Law and Nullum Crimen Sine Lege, Cham, Springer, 2017.
- Scharf, M.P., Customary International Law in Times of Fundamental Change: Recognizing Grotian Moments, Cambridge, Cambridge University Press, 2013.
- Staubach, P.G., The Rule of Unwritten International Law: Customary Law, General Principles, and World Order, New York, NY, Routledge, 2018.
- Thirlway, H., International Customary Law and Codification: An Examination of the Continuing Role of Custom in the Present Period of Codification of International Law, Leiden, Sijthoff, 1972.
- Villiger, M.E., Customary International Law and Treaties: A Manual on the Theory and Practice of the Interrelation of Sources (2nd ed.), The Hague, Kluwer Law International, 1997.
- Berkes, A., “The Formation of Customary International Law By de Facto Regimes” (February 27, 2019), in S. Droubi and J. d’Aspremont (eds.), International Organizations and Non-State Actors in the Formation of Customary International Law, Manchester University Press, 2019. [PDF]
- Brölmann, C.M., “Capturing the Juridical Will of International Organisations (February 25, 2019), in S. Droubi and J. d’Aspremont (eds.), International Organizations and Non-State Actors in the Formation of Customary International Law, Manchester University Press, 2019; Amsterdam Law School Research Paper No. 2019-09; Amsterdam Center for International Law No. 2019-02. [PDF]
- Deplano, R, “The Riddle of Custom: General Assembly Resolutions (November 20, 2018), in S. Droubi and J. d’Aspremont (eds), International Organizations, Non-State Actors, and the Formation of Customary International Law, Manchester University Press, 2019. [PDF]
- Dodge, W.S., “International Comity in the Restatement (Fourth) of Foreign Relations Law” (January 6, 2019). [PDF]
- Gradoni, L., “Un-procedural Customary Law”, Journal of International Dispute Settlement, 10 (2019), No. 2, pp. 175-199. [e-article]
- Nolte, G., “How to Identify Customary International Law?: On the Final Outcome of the Work of the International Law Commission (2018) (June 2019), KFG Working Paper Series, No. 37, Berlin Potsdam Research Group “The International Rule of Law – Rise or Decline?”, June 2019 . [PDF]
- Bourgeois, H., and J. Wouters, “Methods of Identification of International Custom: A New Role for Opinio Juris?”, in R. Pisillo Mazzeschi, and P. De Sena (eds.), Global Justice, Human Rights and the Modernization of International Law, Cham, Springer, 2018, pp. 69-111. [e-article]May, 2019
- Bratspies, R.M., “Reasoning Up to Human Rights: Environmental Rights as Customary International Law”, in J. Knox and R. Pejan (eds.), The Human Right to a Healthy Environment, Cambridge, Cambridge University Press, Forthcoming, August 2018. [PDF]
- Chimni, B.S., “Customary International Law: A Third World Perspective”, American Journal of International Law, 112 (2018), No. 1, pp. 1-46. [e-article]; Related commentaries (7), AJIL Unbound, Vol. 112 - 2018.
- Daugirdas, K., “International Organizations and the Creation of Customary International Law”, University of Michigan Public Law Research Paper No. 597, April 2018. [PDF]
- Gasbarri, L., “Beyond the Either/Or Paradigm in the Formation of Customary International Law by International Organizations”, in J. d’Aspremont and S. Droubi (eds.), International Organizations and the Formation of Customary International Law, Manchester University Press, 2018, Forthcoming. [PDF]
- Fitzmaurice, M., “Customary Law, General Principles, Unilateral Acts”, in E. Sobenes Obregon and B. Samson (eds.), Nicaragua before the International Court of Justice: Impacts on International Law, Cham, Springer, 2018, pp. 247-267. [e-article]
- Fox, G.H., K. Boon, and I. Jenkins, “The United Nations Security Council and the Law of Non-International Armed Conflict: New Evidence of Customary International Law", American University Law Review, 67 (2018), No. 3, pp. 649-731. [PDF]
- Heller, K.J., “Specially-Affected States and the Formation of Custom”, American Journal of International law, 112 (2018), No. 2, pp. 191-243. [e-article] See also Heller (2017)
- Joyner, D., “Why I Stopped Believing in Customary International Law”, Asian Journal of International Law, 9 (2019), No. 1, Forthcoming; U of Alabama Legal Studies Research Paper No. 3228064. [PDF]
- McCall-Smith, K.L. “Treaty Bodies, States and the Shaping of Customary Law”, in S. Droubi and J. d’Aspremont (eds.), International Organizations, Non-State Actors, and the Formation of Customary International Law, Manchester University Press, 2019, Forthcoming. [PDF]
- Okubuiro, J.C., “Application of Hegemony to Customary International Law: An African Perspective”, Global Journal of Comparative Law, 7 (2018), No. 2, pp. 232-271. [e-article]
- Rao, P.S., “The Identification of Customary International Law: A Process that Defies Prescription”, Indian Journal of International Law, (June 2018). [e-article]
- Ryngaert, C.M.J., and D.W. Hora Siccama, “Ascertaining Customary International Law: An Inquiry into the Methods Used by Domestic Courts”, Netherlands International Law Review, (March 2018). [PDF]
- Tan, Y., 2018. The Identification of Customary Rules in International Criminal Law. Utrecht Journal of International and European Law, 34(2), pp.92–110. [e-article] Dec.
- Wood, M., “The Evolution and Identification of the Customary International Law of Armed Conflict”, Vanderbilt Journal of Transnational Law, 51 (2018), No. 3, pp. 727-736. [e-article]
- Yee, S., “AALCO Informal Expert Group’s Comments on the ILC Project on ‘Identification of Customary International Law’: A Brief Follow-up”, Chinese Journal of International Law, 17 (2018), No.1, pp. 187-194. [e-article]
- Arajärvi, N., "The Requisite Rigour in the Identification of Customary International Law", International Community Law Review, 19 (2017), No. 1, pp. 9-46.
- Arajärvi, N., “From the ‘Demands of Humanity’: the Formulation of Opinio Juris in Decisions of International Criminal Tribunals and the Need for a Renewed Emphasis on State Practice”, in B.D. Lepard (ed.), Reexamining Customary International Law, Cambridge, Cambridge University Press, 2017, pp. 189-229.
- Azarova, V., “Adjudicators, Guardians, and Enforcers: Taking the Role of Non-Governmental Organisations in Customary International Law-Making Seriously” (Dec. 2017), in J. d'Aspremont and S. Droubi (eds), Non-State Actors and the Formation of Customary International Law, Manchester, Manchester University Press, 2018, Forthcoming. [PDF]
- Blokker, N., “International Organizations and Customary International Law”, International Organizations Law Review, 14 (2017), No. 1, pp. 1-12.
- Carrillo-Santarelli, N., "The Possibilities and Legitimacy of Non-State Participation in the Formation of Customary Law", International Community Law Review, 19 (2017), No. 1, pp. 98-125.
- d'Aspremont, J., “Non-State Actors and the Formation of International Customary Law: Unlearning Some Common Tropes” (Nov. 2017), in I. Scobbie and S. Droubi (eds), Non-State Actors and the Formation of Customary International Law, Manchester, Manchester University Press, 2018, Forthcoming. [PDF]
- Deplano, R., "Assessing the Role of Resolutions in the ILC Draft Conclusions on Identification of Customary International Law: Substantive and Methodological Issues”, University of Leicester School of Law Research Paper No. 17-05, June 2017. [PDF]
- Droubi, S., "The Role of the United Nations in the Formation of Customary International Law in the Field of Human Rights", International Community Law Review, 19 (2017), No. 1, pp. 68-97. [PDF]
- Droubi, S., “Institutionalisation of Emerging Norms of Customary International Law through Resolutions and Operational Activities of the Political and Subsidiary Organs of the United Nations”, International Organizations Law Review, 14 (2017), No. 2, Forthcoming. [PDF]
- Dumberry, P., “Has the Fair and Equitable Treatment Standard Become a Rule of Customary International Law?”, Journal of International Dispute Settlement, 8 (2017), No. 1, pp. 155-178. [e-article]
- Dupuy, P.-M., “Formation of Customary International Law and General Principles”, in J.E. Viñuales and E. Lees (eds.), Environmental and Energy Law: International Dimensions (Vol. 1), Cheltenham, Edward Elgar Publishing, 2017, pp. 154-171.
- Galindo, G.R.B., and C. Yip, “Customary International Law and the Third World: Do Not Step on the Grass”, Chinese Journal of International Law, 16 (2017), No. 2, pp. 251-270.
- Guliyev, K., “Local Custom in International Law”, International Community Law Review, 19 (2017), No. 1, pp. 47-67.
- Heller, K.J., “Specially-Affected States and the Formation of Custom”, Amsterdam Law School Research Paper No. 2017-45, October 2017. [PDF]
- Henckaerts, M., and E. Debuf, “The ICRC and the Clarification of Customary International Humanitarian Law”, in B.D. Lepard (ed.), Reexamining Customary International Law, Cambridge, Cambridge University Press, 2017, pp. 161-188.
- Henry, E., “Alleged Acquiescence of the International Community to Revisionist Claims of International Customary Law (with Special Reference to the Jus Contra Bellum Regime)”, Melbourne Journal of International Law, 18 (2017), No. 2. [e-article]
- Kelly, J.P., “Customary International Law in Historical Context: The Exercise of Power Without General Acceptance”, in B.D. Lepard (ed.), Reexamining Customary International Law, Cambridge, Cambridge University Press, 2017, pp. 47-85. [PDF]
- Kleinlein, T., “Customary International Law and General Principles: Rethinking Their Relationship”, in B.D. Lepard (ed.), Reexamining Customary International Law, Cambridge, Cambridge University Press, 2017, pp. 131-158.
- McGarry, B., “The Development of Custom in Territorial Dispute Settlement”, Journal of International Dispute Settlement, 8 (2017), No. 2, pp. 339-365.
- Meguro, M., “Customary International Law and Non-State Actors: Between Anthropomorphism and Artificial Unity” (Nov. 2017), in I. Scobbie and S. Droubi (eds), Non-State Actors and the Formation of Customary International Law, Manchester, Manchester University Press, 2018, Forthcoming. [PDF]
- Merkouris, P., "Interpreting the Customary Rules on Interpretation", International Community Law Review, 19 (2017), No. 1, pp. 126-155.
- Miles, C.A., “Thoughts on Domestic Adjudication and the Identification and Formation of Customary International Law, Italian Yearbook of International Law, 27 (2017), pp. 133-149.
- Odermatt, J., “The Development of Customary International Law by International Organizations”, International and Comparative Law Quarterly, 66 (2017), No. 2, pp. 491-511. [e-article]
- Petersen, N., “The Role of Consent and Uncertainty in the Formation of Customary International Law”, in B.D. Lepard (ed.), Reexamining Customary International Law, Cambridge, Cambridge University Press, 2017, pp. 111-130.
- Petersen, N., “The International Court of Justice and the Judicial Politics of Identifying Customary International Law”, European Journal of International Law, 28 (2017), No. 2, pp. 357-385. [e-article]
- Ramsey, M.D., “The Constitution's Text and Customary International Law" (Nov. 2017), San Diego Legal Studies Paper No. 17-313; Georgetown Law Journal, 2018, Forthcoming. [PDF]
- Scoville, R., “Egocentric Bias in Perceptions of Customary International Law”, in H.G. Cohen and T. Meyer (eds.), International Law As Behavior, Cambridge University Press, Forthcoming; Marquette Law School Legal Studies Paper No. 17-10, August 2017. [PDF]
- Tzevelekos, V.P., "Introductory Note: Beyond the Identification of International Customary Rules", International Community Law Review, 19 (2017), No. 1, pp. 1-8.
- Worster, W.T., “The Presumption of Customary International Law: A Case Study of Child Statelessness”, Working Paper, Amsterdam Center for International Law, 2017. [PDF]
- Worster, W.T., “The Contribution to Customary International Law by Territories Under International Administration” (July 2017), in J. d'Aspremont and S. Droubi (eds.), International Organizations and the Formation of Customary International Law, Manchester, Manchester University Press, 2018, Forthcoming. [PDF]
- Yamashita, T., “Investors in the Formation of Customary International Law: An Insight from the 'Futility Exception' to the Local Remedies Rule in Investor-State Arbitration” (Dec. 2017), in I. Scobbie and S. Droubi (eds), Non-State Actors and the Formation of Customary International Law, Manchester, Manchester University Press, 2018, Forthcoming. [PDF].
Periodicals, serial publications
Bibliographies
- Buisman, H.P.C., Customary international law: selective bibliography for the 70th session of the International Law Commission, The Hague, Peace Palace Library, 2017.
- Buisman, H.P.C., Selective Bibliography "Customary International Law & State Practice of France, Germany and the Netherlands", The Hague, Peace Palace Library, 2017.
- Wood, M. (International Law Commission), Fourth Report on Identification of Customary International Law: Addendum (Annex II. Identification of Customary International Law: Bibliography), UN Doc. A/CN.4/695/Add. 1 (2016).
- Wood, M. (International Law Commission), Fifth Report on Identification of Customary International Law: Addendum (Annex II. Identification of Customary International Law: Bibliography), UN Doc. A/CN.4/717/Add. 1 (2018).