Researching Customary International Law, State Practice, and the Pronouncements of States Regarding International Law

By Renu Urvashi Sagreiya

Renu Urvashi Sagreiya is the Reference & Digital Collections Librarian at the Jerene Appleby Harnish Law Library at Pepperdine University Caruso School of Law. She has a B.A. in Sociology & Anthropology with a Minor in Spanish from Agnes Scott College, a J.D. from Drexel University Thomas R. Kline School of Law, and is an M.S.L.S. Candidate at Pennsylvania Western University, Clarion.

Published March/April 2024
(Previously updated by C. Deane in July 2016 and November/December 2018)
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1. Introduction

This research guide is intended to be an introduction to the concept of international custom and its place as a source of international law. The primary focus is on researching state practice and the pronouncements of states regarding international law as evidence of custom. While treaties, state law, and the actions of international organizations can also contribute to customary international law, this guide does not assist with researching these areas. References to some of the excellent research guides already written in these areas are included. The guide introduces the researcher to titles that provide texts of the pronouncements of states regarding international law, both U.S. and international. There are also recommendations for secondary sources and finding aids helpful in describing state practice and in tracking down additional resources. Lastly, a list of additional research guides on customary international law is also provided. These alternate research guides were used extensively in preparation for writing this guide and are highly recommended as additional resources on the subject.

2. What is Customary International Law?

“Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation.”[1] This definition was published in §102 (2) of the Restatement of the Law, Third, Foreign Relations Law of the United States, published by the American Law Institute in 1987. The Restatement’s reporters’ notes for this section state that “No definition of customary law has received universal agreement, but the essence of Subsection (2) has wide acceptance” and goes on to explain various difficulties in defining custom.”[2] Restatement of the Law, Fourth, Foreign Relations Law of the United States was published in 2014. When is state practice considered to be customary international law? The Restatement calls for a two-pronged approach to determining custom requiring both a general and consistent practice and a sense of legal obligation (opinion juris sive necessitates). J.L. Brierly describes it as follows: “Custom in its legal sense means something more than mere habit or usage; it is a usage felt by those who follow it to be an obligatory one. There must be present a feeling that, if the usage is departed from, some form of sanction probably, or at any rate ought to, fall on the transgressor.”[3] Terms such as “a feeling that”, “will probably” and “ought” are difficult to prove. As Mark Janis puts it in his book, An Introduction to International Law, “The determination of customary international law is more an art than a scientific method.”[4] This is a complex and fascinating area of law that is addressed by the many excellent books and articles on customary international law. This guide focuses on finding the resources that provide evidence of international custom. For additional information on the legal issues relating to customary international law, see the following sources:

  • American Law Institute. Restatement of the Law, Third, the Foreign Relations Law of the United States. St. Paul, Minn.: American Law Institute Publishers, 1987. The comments of §102, particularly ‘b’ through ‘e’, are an excellent resource for more information about the concept of custom.
  • Bederman, David J. Custom as a Source of Law. Cambridge [U.K.]; New York: N.Y.: Cambridge University Press, 2010. Part three of this book details the history and application of custom as a source of both private and public international law.
  • Byers, Michael. Custom, Power and the Power of Rules: International Relations and Customary International Law. Cambridge, U.K.; New York: Cambridge University Press, 1999. Part 3 on “The process of customary international law” is particularly useful. (2nd JX2000 B995C96 1999—checked).
  • D’Amato, Anthony A. The Concept of Custom in International Law. Ithaca, N.Y., Cornell University Press, 1971. Written during the Cold War, this is an excellent description of the theory of custom.
  • Kontou, Nancy. The Termination and Revision of Treaties in the Light of New Customary International Law. Oxford: Clarendon Press; New York: Oxford University Press, 1994. A description of what happens when custom and treaties collide.
  • Lepard, B.D., Customary International Law: A New Theory with Practical Applications. Cambridge: Cambridge University Press, 2010. Comprehensive reference text.
  • Lepard, Brian D. Reexamining Customary International Law, New York: Cambridge University Press, 2017. This book provides multiple perspectives on the history and theory of customary international law. They discuss the accepted views on customary international law, to question them. They also analyze the application of customary international law in specific issue areas.
  • Merkouris, Panos, et al. The Theory, Practice, and Interpretation of Customary International Law. Cambridge: Cambridge University Press, 2022. A primer on the history, theory, and interpretation of customary international law, including details of cases appearing in national and international courts.
  • Rosenne, Shabtai. Practice and Methods of International Law. London; New York: Oceana Publications, 1984. Chapter 3 of this text, “Customary International Law,” provides an excellent review of the theory of custom as well as a description of resources that constitute the evidence of custom.
  • Schabas, William. Customary International Law of Human Rights. Oxford: Oxford University Press, 2021. This volume provides an analysis of the fundamental freedoms protected under customary international law.
  • Scharf, Michael P. Customary International Law in Times of Fundamental Change: Recognizing Grotian Moments. Cambridge: Cambridge University Press 2013. This title provides an explanation of customary international law in the introduction.
  • Shaw, M.N., International Law. Cambridge: Cambridge University Press, 2017. In Chapter 3, there is a detailed explanation of customary international law.
  • Staubach, Peter G. The Rule of Unwritten International Law: Customary Law, General Principles, and World Order. New York, NY: Routledge, 2018. This book situates customary international law within the realm of unwritten international law, focusing largely on the judicial interpretation of customary international law in legal practice.
  • Sufyan Droubi, et al. International Organizations, Non-State Actors, and the Formation of Customary International Law. Manchester: Manchester University Press, 2020. Includes an intensive look at the controversies within the case law that forms customary international law.
  • Thirlway, H. W. A. International Customary Law and Codification; An Examination of the Continuing Role of Custom in the Present Period of Codification of International Law. Leiden: A. W. Sijthoff, 1972. An interesting, if dated, text on custom and the codification of international law.
  • Wallace, Rebecca, Fraser Janeczko & Karen Wylie. Nutshells International Law. London: Sweet & Maxwell, 2013. Chapter 3 gives a brief, clear overview of the sources of international law.
  • Wolfke, Karol. Custom in Present International Law, 2d. Dordrecht, Boston: Martinus Nijhoff Publishers, 1993. Although somewhat dated (it was not significantly altered since the 1964 edition), this volume provides a good overview of customary international law, with extensive footnotes. The bibliography is more current and includes helpful references to both U.S. and non-U.S. works on customs.

3. Custom as a Source of Law

Customary law is listed as a source of international law in the Restatement (Third) of Foreign Relations Law in section 102(1)(a), along with international agreements and general principles:

§ 102 Sources of International Law states that “(1) A rule of international law has been accepted as such by the international community of states: (a) in the form of customary law; (b) by international agreement; or (c) by derivation from general principles common to the major legal systems of the world.”[5]

The Restatement’s description of the sources of international law is derived in part from the Statute of the International Court of Justice,[6] described in the Restatement as an “authoritative statement of the ‘sources’ of international law.”[7] Article 38 of the Statute describes what the court should consider in making decisions by international law: Article 38 of the ICJ states that “(1) The Court, whose function is to decide by international law such disputes as are submitted to it, shall apply:

(a) international conventions, whether general or particular, establishing rules expressly recognized by the contesting states; (b) international custom, as evidence of a general practice accepted as law; (c) the general principles of law recognized by civilized nations; (d) subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.”[8]

The concept of customary international law is briefly explained by ICJ Judge Sir Christopher Greenwood in his video lecture Sources of International Law, available in the Audiovisual Library of International Law. This concept is more fully explained by attorney and law professor, Mr. Anthony D’Amato in his video lecture The Sources of International Law – Part 2: Customary International Law also available in the Audiovisual Library of International Law.

4. Evidence of Customary International Law

The Restatement describes the evidence of international law in §103. Of particular interest to the researcher of custom is §103(2)(d) which describes the evidence of pronouncements of states: § 103 Evidence of International Law

(1) Whether a rule has become international law is determined by evidence appropriate to the particular source from which that rule is alleged to derive (§ 102). (2) In determining whether a rule has become international law, substantial weight is accorded to (a) judgments and opinions of international judicial and arbitral tribunals; (b) judgments and opinions of national judicial tribunals; (c) the writings of scholars; (d) pronouncements by states that undertake to state a rule of international law, when such pronouncements are not seriously challenged by other states.[9]

The Restatement comments state, “Thus, for customary law, the ‘best evidence’ is the proof of state practice, ordinarily by reference to official documents and other indications of governmental action.”[10] In Principles of Public International Law, Ian Brownlie lists the following sources as evidence of custom: “The material sources of custom are very numerous and include the following: diplomatic correspondence, policy statements, press releases, the opinions of official legal advisers, official manuals on legal questions, e.g. manuals of military law, executive decisions, and practices, orders to naval forces, etc., comments by governments on drafts produced by the International Law Commission, state legislation, international and national judicial decisions, recitals in treaties and other international instruments, a pattern of treaties in the same form, the practice of international organs, and resolutions relating to legal questions in the United Nations General Assembly.”[11]

4.1. State Practice

The practice of states must be both general and consistent and combined with a state’s sense of a legal right or a legal obligation. The Encyclopaedic Dictionary of International Law (3 ed., 2009) edited by John P. Grant and J. Craig Barker has a definition of state practice that includes references to authoritative International Law treatises. For an extensive, but not exhaustive list of sources of state practice, see Treves T. Customary international law, Part C7 The Sources of Knowledge of Practice, Max Planck Encyclopedia (Oxford University Press, 2006).

4.2. Pronouncements of States

Official pronouncements by states that undertake to state a rule of international law are one of the best places to find evidence of international custom as they often describe both prongs of the definition of custom, the general and consistent practice of the state and an explicit description of the state’s sense of legal obligation. Most of this guide is dedicated to finding these resources.

4.3. Treaties

According to the Restatement, “International agreements constitute practice of states and as such can contribute to the growth of customary law under Subsection (2).”[12] There are many excellent guides to treaty research available. See for example the GlobaLex article titled UPDATE: An Introduction to Sources for Treaty Research authored by Mark Engsberg and Kwanghyuk (David) Yoo (May/June 2021), the Arthur W. Diamond Law Library Treaties Research Guide (authored by Nam Jin Yoon) (last updated in 2022), or the Alyne Queener Massey Law Library research guide titled Treaty Research Guide (authored by Emily Pavuluri and last updated in 2024).

4.4. State Law

A country’s laws (legislation, court decisions, regulations, etc.) are sources of state practice to the extent that they might seek to describe the country’s practices and obligations under international law. The many excellent research articles, available in the foreign law section of GlobaLex and the Foreign Law Guide, edited by Marci Hoffman, serve as resources for those researching the domestic laws of all countries. Often you can find foreign law research guides among the research guide offerings at high-ranking law schools.[13] In addition, the Library of Congress’s foreign research guides are updated frequently.

4.5. International Organizations and International Courts

As described in the Restatement, “The practice of states that builds customary law takes many forms and includes what states do in or through international organizations.”[14] In particular, the Restatement refers to resolutions and other documents of the United Nations. The decisions of international courts likewise are a good source of information. Research guides and articles to the United Nations and other international organizations describe the documents of these organizations. See for example:

UN’s Dag Hammarskjöld Library Research Guides

5. Pronouncements of States—Resources

When looking for pronouncements of states describing international law, one can research the publications of the state itself. Alternatively, using the many excellent digests and secondary sources available will provide the researcher with explanatory notes regarding the pronouncements as well as good citations to these documents and sometimes reprints of the texts themselves. The following sections describe both of these types of resources.

5.1. Pronouncements of States—The United States of America

In the United States, the State Department publications include pronouncements that undertake to state the rules of international law and as such are excellent resources for the evidence of custom. Examples of these documents include memorandums, letters from legal advisors, U.S. briefs, position papers, and remarks. Today, the State Department and the Office of the Legal Adviser provide many of these resources on their websites. For more information on researching foreign relations of the United States, there are many good U.S. Foreign Policy Research Guides, such as:

While the resources in this section can be excellent resources, it is sometimes more practical to use resources that have already compiled and analyzed domestic documents that relate to international law, such as the digests and secondary sources described in the subsequent sections. Following is a list of resources for U.S. diplomatic papers in reverse chronological order:

  • Website of the Department of State: The website of the State Department of the United States currently includes the types of documents previously published in the paper sources listed below, including speeches, briefings, testimony, and treaty actions. Currently, speeches go back to 2017, but you can use the archives link to go back as far as 1993. Treaty actions go back to 1997.[15]
  • U.S. Department of State Dispatch. Bureau of Public Affairs: Washington, D.C.; Office of Public Communication, 1990-1999. Weekly, 1990-1996; Monthly, 1997-1999. Updates on foreign relations in the U.S. Although the publication has ceased, the type of information provided in Dispatch is now published on the website of the Department of State. Diplomatic papers are accessible via the full-text search at GovInfo.
  • Department of State Bulletin. Washington, D.C.: Office of Public Communication, Bureau of Public Affairs, 1939-1989. For years, this publication was the official record of foreign policy in the U.S. Foreign Relations of the United States. Washington, D.C.: Government Publishing Office, 1861-. The formal, historical record of the State Department’s work. The most recent volumes cover up to the early seventies. An excellent historical guide to this publication is available on the website of the Office of the Historian, U.S. Department of State. The website also includes the full text.
  • America State Papers: Documents, Legislative and Executive, of the Congress of the United States. Washington, D.C. Gales and Seaton, 1832-1861. Covers 1789-1832. Published by the U.S. Congress. The American State Papers are available online at the Library of Congress website, which describes the publications as follows: “The American State Papers, comprising a total of thirty-eight physical volumes, contain the legislative and executive documents of Congress during the period 1789 to 1838.” The website is both browsable and searchable.

5.2. Digest of U.S. Practice in International Law

Over the years, an official digest of U.S. practice in international law has been published in various forms, with different authors, sometimes annually. These digests include editorial descriptions of international law in various areas as well as selections of official U.S. documents that are evidence of customary international law. Published by the Department of State, authorship has changed over the years, and recent issues are published with the International Law Institute under the auspices of the Department of State, Office of the Legal Advisor. The Digest is one of the best sources for determining the official State Department’s view of international law. On its website, the State Department describes this publication as follows: “The Office of the Legal Adviser publishes the annual Digest of United States Practice in International Law to provide the public with a ready source of current information on the views and practice of the Government of the United States in the arena of public and private international law.” Many documents cited in editions of the Digest since 1989 are available on the website of the Department of State. Following is a list of these digests:

  • 1877: Cadwalader, John L. United States Department of State. Digest of the Published Opinions of the Attorneys-General, and the Leading Decisions of the Federal Courts: With Reference to International Law, Treaties, and Kindred Subjects. Washington, D.C.: Government Printing Office, 1877. 1 volume.
  • 1886: Wharton, Francis, Ed. A Digest of the International Law of the United States, taken from Documents Issued by Presidents and Secretaries of State, and from Decisions of Federal Courts and Opinions of Attorneys-General. Washington, D.C.: Government Printing Office, 1887. 3 volumes.
  • 1906: Moore, John Bassett. A Digest of International Law As Embodied In Diplomatic Discussions, Treaties and Other International Agreements, International Awards, The Decisions Of Municipal Courts, And The Writings Of Jurists. Washington, D.C.: Government Printing Office, 1906. 8 volumes.
  • Covering 1906-1940: Hackworth, Green Haywood. Digest Of International Law. Washington, D.C.: Government Printing Office, 1940-44. 8 volumes.
  • Covering 1940-1960 (volumes published later in the series covered up to 1970): Whiteman, Marjorie M. Digest of International Law. Washington, D.C.: U.S. Department of State, 1963-73. 15 volumes.
  • Covering 1973-1980: Digest of United States Practice in International Law. Washington, D.C.: Office of the Legal Adviser, Department of State, 1974-1986. Annual. Prepared by: Arthur W. Rovine, 1973-1974; Eleanor C. McDowell, 1975-1976; John A. Boyd, 1977; Marian L. Nash, 1978-1980.
  • Covering 1981-1988: Cumulative Digest of United States Practice in International Law. Washington, D.C.: Office of the Legal Adviser, Dept. of State, 1993-1995. 3 volumes. Editor: Marian Nash (Leich).
  • Covering 1989-: Digest of United States Practice in International Law. Washington, D.C.: International Law Institute, 2001-. The volumes were not published in sequence but began publication in 2001 with the volume for 2000. Earlier volumes were published retrospectively to fill in the gap between 2000 and the Cumulative Digest (covering 1981-1988). Many documents cited in editions since 1989 are available on the website of the Department of State. The introduction notes, “The State Department publishes the official version of the Digest exclusively online to make U.S. views on international law more quickly and readily accessible to our counterparts in other governments, and to international organizations, scholars, students, and other users, both within the United States and around the world.”
  • Covering 1989-1990, Mary S. Pickering, Sally J. Cummins, and David P. Stewart, editors, published in 2003. 1 volume.
  • Covering 1991-1999, Sally J. Cummins and David P. Stewart, editors, published in 2005. 2 volumes.
  • Covering 2000, Sally J. Cummins and David P. Stewart, editors, published in 2001. 1 volume.
  • Covering 2001, Sally J. Cummins and David P. Stewart, editors, published in 2002. 1 volume.
  • Covering 2002, Sally J. Cummins and David P. Stewart, editors, published in 2003. 1 volume.
  • Covering 2003, Sally J. Cummins and David P. Stewart, editors, published in 2004. 1 volume.
  • Covering 2004, Sally J. Cummins, editor, published in 2006. 1 volume.
  • Covering 2005, Sally J. Cummins, editor, published in 2007. 1 volume.
  • Covering 2006, Sally J. Cummins, editor, published in 2007. 1 volume.
  • Covering 2007, Sally J. Cummins, editor, published in 2008. 1 volume.
  • Covering 2008, Elizabeth R. Wilcox, editor, published in 2010. 1 volume.
  • Covering 2009, Elizabeth R. Wilcox, editor, published in 2011. 1 volume.
  • Covering 2010, Elizabeth R. Wilcox, editor, published in 2011. 1 volume.
  • Covering 2011, CarrieLyn D. Guymon, editor, published in 2012. 1 volume. The official version was published exclusively online for the first time.
  • Covering 2012, CarrieLyn D. Guymon, editor, published in 2013. 1 volume. The official version is published exclusively online.
  • Covering 2013, CarrieLyn D. Guymon, editor, published in 2014. 1 volume. The official version is published exclusively online.
  • Covering 2014, CarrieLyn D. Guymon, editor, published in 2015. 1 volume. The official version is published exclusively online.
  • Covering 2015, CarrieLyn D. Guymon, editor, published in 2016. 1 volume. The official version is published exclusively online.
  • Covering 2016, CarrieLyn D. Guymon, editor, published in 2017. 1 volume. The official version is published exclusively online.
  • Covering 2017, CarrieLyn D. Guymon, editor, published in 2018. 1 volume. The official version is published exclusively online.
  • Covering 2018, CarrieLyn D. Guymon, editor, published in 2019. 1 volume. The official version is published exclusively online.
  • Covering 2019, CarrieLyn D. Guymon, editor, published in 2020. 1 volume. The official version is published exclusively online.
  • Covering 2020, CarrieLyn D. Guymon, editor, published in 2021. 1 volume. The official version is published exclusively online.
  • Covering 2021, CarrieLyn D. Guymon, editor, published in 2022. 1 volume. The official version is published exclusively online.
  • Covering 2022, Tiffany Holloman, editor, published in 2023. 1 volume. The official version is published exclusively online.

5.3. Secondary Sources—United States

Secondary sources are sometimes the best way to find descriptions of a state’s practice in international law. Also, they usually provide excellent citations to the documents themselves and sometimes excerpts or texts of the documents. Following is a list of secondary sources on the practice of the U.S. in international law:

  • American Journal of International Law, “Contemporary Practice of the United States relating to International Law.” Quarterly, beginning in volume 53, 1959. From the Editor’s note of the first issue of this feature, “Criteria for the selection of materials may vary with experience, but the guiding purpose will be to select materials which reveal contemporary practice by the United States in invoking and applying principles, rules and procedures of international law or policies relating thereto.”[16]
  • Belia, Anthony J. and Clark, Bradford R. The Law of Nations and the United States Constitution. New York: Oxford University Press, 2017. The book analyzes customary international law through the lens of the Constitution.
  • American Law Institute. Restatement of the Law, Third, the Foreign Relations Law of the United States. St. Paul, Minn.: American Law Institute Publishers, 1987. An excellent and widely respected statement on international law. Note—the foreword explicitly states that it is “in no sense an official document of the United States.”[17]
  • Christol, Carl Q. International Law and U.S. Foreign Policy. Lanham, Md: University Press of America, 2007.
  • Hyde, Charles Cheney. International Law: Chiefly as Interpreted and Applied by the United States, 2nd rev. ed. Boston: Little, Brown, 1945. AND Hyde, Charles Cheney. International Law: Chiefly as Interpreted and Applied by the United States. Boston: Little, Brown, and Company, 1922. In two volumes in 1922 and three volumes in 1945, these are excellent sources for historical research. Both include extensive footnotes.
  • Murphy, Sean D. United States Practice in International Law. Cambridge, U.K.; New York: Cambridge University Press, 2002, 2006. Volume 1 covers 1999-2001 and volume 2 covers 2002-2004. This is a very useful analysis of the United States in the arena of international law. It includes extracts of some documents and extensive citations to others, both domestic and international. It includes helpful tables of U.S. cases, U.S. statutes, and U.S. treaties.
  • Paust, Jordan J. International Law as Law of the United States, 2nd ed. Durham, N.C.: Carolina Academic Press, 2003. The first chapter of this book, “Customary International Law and Forms of Incorporation” is an excellent and recent overview of custom from a U.S. perspective. It includes a rich ‘notes’ section—50 pages of citations to cases, statutes, secondary sources, and more.
  • 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.
  • Romano, Cesare. The Sword and the Scales: The United States and International Courts and Tribunals. Cambridge: Cambridge University Press, 2009.
  • Scharf, Michael P, and Paul R. Williams. Shaping Foreign Policy in Times of Crisis: The Role of International Law and the State Department Legal Adviser. Cambridge U.K.: Cambridge University Press, 2010.
  • Scoville, R.M., 2015. Finding Customary International Law. Iowa L. Rev., 101, p.1893. (Explains how U.S. judges find customary international law.)
  • Van, der V. J. D. Implementation of International Law in the United States. Frankfurt am Main: P. Lang, 2010.

5.4. Pronouncements of Non-U.S. States

When researching state practice from the perspective of countries other than the United States, the same principles apply as in U.S. research. The best resources will usually be publications of a government’s State Department, Ministry of Foreign Affairs, or Ministry of External Affairs. Many foreign ministries are putting a significant amount of information on their websites. Alternatively, you can use the name of the ministry as the author in a catalog search or limit an internet search. A selective list of foreign ministries and their websites is included below, but a Google search for the country name and the word foreign affairs will usually bring up the appropriate agency for any country:

5.5. International Yearbooks

An excellent way to find information about state practice in other jurisdictions is by using yearbooks on international law. Many states publish such a yearbook. They are unofficial, often published by a national law society, and include articles about international law and state practice. They also include citations to official documents and sometimes include indexes and full-text documents. They often have bibliographies of books and articles published in that country on international law. Many are published (or translated into), at least in part, in English and are therefore a great resource for those who cannot read the vernacular. International law yearbooks can be found in a library catalog by searching for both “yearbook” and “year book” (as one word or two), and the term “international law.” Occasionally, publications including the classic content of an international law yearbook may be called by another name (annual, etc.). While subject headings in catalogs and indexes often include the term “yearbook” or “year book,” in case they do not it is also useful to search catalogs for the term in the vernacular: “annuaire,” “anuario,” “jahrbuch,” etc. A selective list of yearbooks is included below.

5.6. State Yearbooks

  • Annuaire Français de Droit International. Paris: Centre National de la Recherche Scientifique, 1955-. Articles, reviews of jurisprudence, treaties, and state practice. Includes an extensive bibliography.
  • The Australian Yearbook of International Law. Canberra, Australia: Centre for International and Public Law, Faculty of Law, The Australian National University. Sydney: Butterworths, 1966-. Includes articles and reviews of books, cases, legislation, state practice, and treaties. Also includes a survey of recent developments in Australian private international law.
  • The British Yearbook of International Law. London: H. Frowde, Hodder and Stoughton, 1921-. Includes articles, book reviews, and a review of British decisions and United Kingdom materials on international law.
  • The Canadian Yearbook of International Law. Annuaire Canadien de Droit International. Vancouver: University of British Columbia Press, 1963-. Includes articles, book reviews, and an index of cases. Articles in both French and English, with a summary in the other language.
  • Chinese (Taiwan) Yearbook of International Law and Affairs. Baltimore, Md.: Occasional Papers/Reprints Series in Contemporary Asian Studies, Inc. for the Chinese Society of International Law, and the Chinese (Taiwan) Branch of International Law Association, 1982-. Includes articles, recent developments, book reviews, a list of treaties, information on diplomatic missions, a bibliography, and treaty and case indexes in English. Includes some documents in full text.
  • Ethiopian Yearbook of International Law. New York: Springer: 2017-. Includes scholarship and legal analysis. In English.
  • The Finnish Yearbook of International Law. Helsinki: Ius Gentium Association, 1990-. Includes articles, book reviews, dissertations, and recent developments and state practice. In English.
  • German Yearbook of International Law. Jahrbuch für Internationales Recht. Berlin: Duncker & Humblot, 1976-. Includes articles, reports, and book reviews in German and English.
  • The Irish Yearbook of International Law. London: Bloomsbury, 2006-. Includes articles, reports, and book reviews in English and Gaelic.
  • Hungarian Yearbook of International Law and European Law. The Hague: Eleven International, 2013-. Includes articles, materials, and other documentation.
  • The Italian Yearbook of International Law. The Hague; New York: Kluwer Law International, 1975-. Includes articles, notes, comments, and a review of Italian practice relating to international law, including judicial decisions, treaties, and legislation.
  • The Japanese Annual of International Law. Tokyo: Japan Branch of the International Law Association, 1957-. Includes articles, a Japanese digest of international law, book reviews, judicial decisions, treaties, and legislation, some in full text. In English. In 2008, it was announced by the association that the title of the publication would be changed to the “Japanese Yearbook of International Law,” and that the publication would be doubled in size to also feature contributions on general topics of international law written by individuals including foreign contributors, to offer a greater variety of perspectives.
  • Netherlands Yearbook of International Law. The Hague: T.M.C. Asser Press, 1970-. Published jointly with the Netherlands International Law Review and under the auspices of the Stichting T.M.C. Asser Instituut, Institute for Private and Public International Law, International Commercial Arbitration and European Law, The Hague. Includes articles and reviews of state practice, treaties, municipal legislation, judicial decisions, and literature relating to international law.
  • New Zealand Yearbook of International Law. Christchurch, N.Z.: International Law Group, School of Law, University of Canterbury, 2004-. Articles and commentary on New Zealand and international law. Also covers the South Pacific, Antarctica, and the Southern Ocean.
  • Nigerian Yearbook of International Law, Cham, Switzerland, 2018-. Includes articles and reports. In English.
  • The Palestine Yearbook of International Law. Nicosia, Cyprus: Al-Shaybani Society of International Law, 1984-. Published in cooperation with the Bizreit University Institute of Law, under whose auspices it is edited. Includes articles and reports on judicial decisions and legislation in Israel. Includes the full text of some documents and book reviews. In English.
  • The Philippine Yearbook of International Law. Manila: The Philippine Society of International Law, 1969-1989. Volumes for 2019, 2020, and 2021 were published online by the University of the Philippines College of Law. Includes articles and the full text of some documents.
  • Polish Yearbook of International Law. Wroclaw: Zaklad Narodowy Im. Ossolinskich. 1967-. Articles, book reviews, and a bibliography. Includes contributions in English, French, and Polish.
  • Rossiiskii Ezhegodnik Mezhdunarodnogo Prava. Russian Year-book of International Law. St. Petersberg: Sotsialno-kommercheskaia Firma “Rossiia-Neva”, 1994-. Published by the Russian Association of International Law. Includes articles and reports. The table of contents is also available in English.
  • Schweizerisches Jahrbuch für Internationales Recht. Annuaire Suisse de Droit International. Zürich: Polygraphischer Verlag, 1944-1990. Reviews of public and private international law. Includes articles in French, German and English. Ceased upon publication of Schweizerische Zeitschrift für Internationales und Europäisches Recht. Revue Suisse de Droit International et de Droit Européen. Zü,rich: Schulthess Polygraphischer Verlag, 1991-.
  • The Singapore Year Book of International Law. Singapore: Faculty of Law, National University of Singapore: Gaunt Law Book Pub., 2004-. Includes speeches, articles, notes, and features.
  • South African Yearbook of International Law. Suid-Afrikaanse Jaarboek vir Volkereg. Pretoria: VerLoren Van Themaat Centre for International Law, University of South Africa. 1975-. Includes notes and comments, articles on foreign judicial decisions, South African judicial decisions, foreign policy, international events, treaties, and literature.
  • Spanish Yearbook of International Law. Dordrecht; Boston: Martinus Nijhoff Publishers, 1994-. Edited by the Asociación Española de Profesores de Derecho Internacional y Relaciones Internacionales and translated into English. Includes articles and reviews of diplomatic practice, treaties, municipal legislation, judicial decisions, and literature relating to international law.

5.7. Multi-Jurisdictional Yearbooks

  • African Yearbook of International Law. Annuaire Africain de Droit International. The Hague; Boston: Kluwer Law International, 1993-. Published under the auspices of the African Foundation for International Law, it includes articles, notes, comments, and book reviews in French and English.
  • Annuaire Européen. European Yearbook. The Hague: Nijhoff, 1955-. Published under the auspices of the Council of Europe, it covers various European organizations.
  • Asian Yearbook of International Law. The Hague; Boston: Kluwer Law International, 1993-. Includes articles and reviews of state practice in China, India, Iran, Japan, Korea, Nepal, Pakistan, the Philippines, Tajikistan, and Thailand. Includes some documents in full text in English.
  • Baltic Yearbook of International Law. The Hague; New York: Kluwer Law International 2002-. Published in English, it includes articles and materials on international law. Covers Estonia, Latvia, and Lithuania.
  • The Global Community Yearbook of International Law. London: Oxford University Press, 2001-. Includes articles and materials on international law worldwide.

5.8. Secondary Sources—Other Jurisdictions

There are many excellent secondary sources on customary international law from other jurisdictions. This section covers sources that include multiple jurisdictions. These sources can be used to find titles and citations for country-specific sources. Alternatively, searches for “international law” combined with terms like “practice,” “applied,” “digest,” or “interpreted” and the name of the country in question can be used in catalogs and other resources for finding country-specific titles.[18]

  • Gaebler, Ralph and Alison A. Shea, eds. Sources of State Practice in International Law. Leiden, the Netherlands; Boston: Brill-Nijhoff, 2014-. An excellent resource for sources of state practice. Chapters cover the following countries: Argentina, Australia, Austria, Belarus, Belgium, Canada, China, Czech Republic, Denmark, France, Germany, Ghana, India, Ireland, Israel, Italy, Japan, Kenya, Republic of Korea, Mexico, Nigeria, Norway, Romania, Russia, Singapore, Sweden, Swiss Confederation, the United Kingdom, and the United States of America.
  • United Nations. Secretariat. Ways and Means of Making the Evidence of Customary International Law More Readily Available: Preparatory Work within the Purview of Article 24 of the Statute of the International Law Commission: Memorandum Submitted by the Secretary-General. Lake Success: United Nations, General Assembly, International Law Commission, 1949. U.N. Document # A/CN.4/6. A memorandum from the U.N. Secretariat on the evidence of customary international law. While quite dated, this publication includes a useful list of compilations and digests of state practice, covering over 20 countries. This is useful for historical research.

6. Additional Research Guides and Scholarship on Customary International Law

6.1. Online Research Guides

  • Winer, Anthony S., Mary Ann E. Archer and Lyonette Louis-Jacques, International Law Legal Research, Carolina Academic Press, 2013. Chapter 8, entitled Customary International Law gives an overview of customary international law and legal research.
  • Hoffman, Marci and Mary Rumsey, International and Foreign Legal Research: A Coursebook, 2nd ed. Leiden: Brill | Nijhoff. 2012. Chapter 7, on Customary International Law, provides a research strategy and some sources of diplomatic documents. The foreign law research guidance in Chapter 8 should also be useful for researching customary international law.
  • Schaffer, Ellen G. and Randall J. Snyder, eds. Contemporary Practice of Public International Law. Dobbs Ferry, N.Y.: Oceana Publications, 1997. This publication is the result of the fifth of a series of American Association of Law Libraries’ sponsored institutes on foreign, comparative, and international law and legal research held in 1996. Several chapters address customary international law. Of particular interest is Ralph Gaebler’s chapter on “Conducting Research in Contemporary International Law.”

6.3. Scholarship

  • Ryngaert, C. M., & Siccama, D. W. H. (2018). Ascertaining Customary International Law: An Inquiry into the Methods Used by Domestic Courts. Netherlands International Law Review, 1-25.
  • Chimni, B. S. (2018). Customary International Law: A Third World Perspective. American Journal of International Law, 112(1), 1-46.
  • Blokker, N. (2017). International Organizations and Customary International Law. international organizations law review, 14(1), 1-12.

[1] American Law Institute. Restatement of the Law, Third, the Foreign Relations Law of the United States. St. Paul, Minn.: American Law Institute Publishers, 1987. §102(2). [hereinafter referred to as Restatement].

[2] Id. §102, Reporters’ Notes, 2.

[3] Brierly, J. L. The Law of Nations: An Introduction to the International Law of Peace, 6th ed. Oxford; New York: Oxford University Press, 1963. p. 59.

[4] Janis, Mark W. An Introduction to International Law, 4th ed. New York, Aspen Publishers, 2003. p. 44.

[5] Id. §102(1)(a).

[6] Statute of the International Court of Justice. 3 Bevans 1179; 59 Stat. 1031; T.S. 993; 39 AJIL Supp. 215 (1945).

[7] Restatement, §102, Reporters’ Notes 1.

[8] Statute of the International Court of Justice, Article 38, § 1.

[9] Restatement, §103.

[10] Restatement, §103, comment a.

[11] Brownlie, Ian. Principles of Public International Law, 6th ed. Oxford; New York: Oxford University Press, 2003. p. 6.

[12] Restatement, §102, comment i.

[13] Bodleian Libraries Law: Jurisdictions. Written by Ruth Bird, Elizabeth Wells, et al. Last Updated: December 21, 2023.

[14] Restatement, §102, Reporters’ Note 2.

[15] The Federal Depository Library at the Richard J. Daley Library, University of Illinois at Chicago maintains a database that includes the Secretary of State speeches as far back as 1993.

[16] See 53 Am. J. Int’l. L. (1959) at 896.

[17] Restatement, Forward, p. IX.

[18] For examples, see Kindred, Hugh M. and Phillip M. Saunders, eds. International Law, Chiefly As Interpreted and Applied in Canada. 7th ed. Toronto: Emond Montgomery Publications, 2006; and “Völkerrechtliche Praxis der Bundesrepublik Deutschland” in Zeitschrift für Ausländisches Öffentliches Recht und Völkerrecht. Stuttgart: W. Kohlhammer, 1929-. Also available online at the website of the Max Planck Institute for Comparative Public Law from Volume 1, 1929 forward.