Sources of International Law

The generally recognized authoritative statement on the sources of international law is the Statute of the International Court of Justice (ICJ), Article 38, which specifies that the Court, in deciding disputes, shall apply:

  • international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
  • international custom, as evidence of a general practice accepted as law;
  • the general principles of law recognized by civilized nations;
  • 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.

The first three of these--treaties, custom, and principles of law--are sometimes referred to by lawyers and librarians with a common law background as "primary sources" of international law. The last two--judicial decisions and the teachings of publicists--are sometimes referred to as "secondary sources" or evidence of international law rules.

Note that case law is considered only a "subsidiary means." Even the decisions of the ICJ itself do not create binding precedent.

The decision of the Court has no binding force except between the parties and in respect of that particular case. (Article 59)

Note, also, that "teachings of publicists" now include the work of organizations such as the International Law Commission and private institutions.

More recent discussions of the sources of international law, recognizing the growing role of international organizations, include the resolutions and other acts of international governmental organizations, such as the United Nations, as sources or evidence of international law.

For further reading:

  • Buergenthal, Thomas and Sean D. Murphy, Public International Law in a Nutshell (4th ed, 2007), pages 18-34. I,MON KZ 3410 .B84 2007 (ON RESERVE).
  • Aust, Anthony, Handbook of International Law (2nd ed., 2010), pages 5-11. I,MON KZ 3410.A97 2010
  • Brownlie, Ian. Principles of Public International Law, (7 th ed., 2008), pages 3-29 I,MON KZ 3225 .B76A37 2008

a) International Conventions (Treaties)

Treaties are the single most important source of international law. For guides to treaty research and collections of treaties, see our web page for Treaties and Treaty Research . Some major treaty collections are:

b) International custom

The leading book on the theory of custom in international law is:
  • D’Amato, Anthony A. The Concept of Custom in International Law. Cornell University Press, 1971.
    I,MON JX 3110 .D3C6 1971
Two excellent research guides are:

The following is a selective list of documentary sources from which evidence of state practice may be sought:

United States Practice in International Law--Official Sources:

  • Digests of practice in international law (United States Department of State).
    Note: The following are available in the HeinOnline Foreign and International Law Resources Database, as well as in hard copy.
    • Digest of United States Practice in International Law. Annual. Covers 1989/90-
      Also available at U.S. Dept. of State Web site
      I,MON JX 237 .D56
    • Cumulative Digest of United States Practice in International Law. Editor: Marian Nash (Leich) 3 vols. Covers 1981 to 1988.
      I,MON JX 237 .D55
    • Digest of United States Practice in International Law. Annual, 1973-1980 .
      I,MON JX 237 .D54
    • Whiteman, Marjorie M. Digest of International Law. Washington, D.C., U.S. Government Printing Office, 1963-1973. 15 vols. Covers 1940-1960, primarily. Coverage is irregular. Volumes were published out of order; those published later include materials from the early 1970s.
      I,MON JX 237 .W55
    • Hackworth, Green Hayward. Digest of International Law. Washington, D.C., U.S. Government Printing Office, 1940-1944. 8 vols. Covers 1906 to 1939.
      I,MON JX 237 .H3
    • 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., US. Government Printing Office, 1906. 8 vols.
      I,MON JX 237 .M7 1906
    • Wharton, Francis. A Digest of the International Law of the United States, Taken FromDocuments Issued by Presidents and Secretaries of State, and From Decisions of Federal Courts and Opinions of Attorneys-General. 2nd ed. Washington, D.C.: U.S. Government Printing Office, 1886. 3 vols
      I,MON JX 237 .W5 1887
    • Cadwalader, John L. Digest of the Published Opinions of the Attorneys-General and of the Leading Decisions of the Federal Courts, with Reference to International Law, Treaties, and Kindred Subjects. Washington, D.C., G.P.O. 1 vol.
      I,MON JX 237 .C33 1877
  • Foreign Relations of the United States. United States Department of State.
    Available from U.S. Dept. of State and HeinOnline Available from anywhere to all members of the Northwestern University community Issued in various series. This multivolume set, arranged by time period and geographic area, is the official documentary record. Supplementary materials are available on microfilm.
    Int 90 P21 through Int 90 P2142
  • American Foreign Policy: Current Documents.
    Annual 1956-1967 and 1981-1990. Avaialble in HeinOnline.

United States Practice in International Law--Unofficial Sources:

  • Restatement of the Law, Third... the Foreign Relations Law of the United States. American Law Institute, 1986. 2 vols.
    MON KF 4651 .R48 1987 v. 1-3 and suppl. 2007 ON RESERVE. Available in HeinOnline Available from anywhere to all members of the Northwestern University community.
  • American Journal of International Law, published by the American Association of International Law (ASIL), includes a section on "Contemporary Practice of the United States Relating to International Law" in each issue.
    I,PER K 1 .M47677
  • Murphy, Sean D. United States Practice in International Law. Cambridge University Press, 2002. v. 1. 1999-2001 -- v. 2. 2002-2004
    I,MON KZ 3410 .M87 2002

Countries Other than the United States :

  • Yearbooks of International Law: Yearbooks of international law often contain information on current state practice.
    Note: backfiles of many yearbooks of international law in PDF format are found in HeinOnline in the Foreign & International Law Resources Database, under Part I: International Law Yearbooks and Periodicals. Some examples are:
    • Annuaire Français de Droit International
      I,MON K 1 .N564
    • British Year Book of International Law
      I,PER K 2 .R638
    • Canadian Yearbook of International Law
      I,PER K 3 .A478
    • Japanese Annual of International Law
      I,PER K 10 .A62
    • South African Yearbook of International Law
      I,PER K 23 .O349
    • Spanish Yearbook of International Law
      I,PER K 23 .P3

Governmental Web sites, particularly those of foreign ministries, can be useful sources of information on the current international practice of the given country.

c) General Principles of Law

Article 38(c) is generally considered to refer to the rules of municipal (domestic) legal systems, and as such is often considered to fall within the purview of comparative law. See our Web page Foreign and Comparative Legal Resources

d) Judicial Decisions

The decisions of international courts and tribunals, as well as those of municipal (domestic) courts may play a "subsidiary" role in helping to determine rules of international law. See our page for International Courts and Tribunals. For the case law of foreign countries please see the various guides included in our page for Foreign and Comparative Law by Country or Region

  • Oxford Reports on International Law in Domestic Courts Available from anywhere to all members of the Northwestern University community Selective, annotated reports, full-text searchable.
  • International Law Reports. Includes decisions of international tribunals, international arbitrations, and decisions of national tribunals. Each volume includes a digest of cases, table of treaties, and index. Consolidated index volumes have been published at various intervals.
    Int 003 A61

e) "Teachings of the most highly qualified publicists" (scholarly writing)

This "subsidiary means" of determining international law rules, per Article 38(d) of the statute of the International Court of Justice, is interpreted to mean significant scholarly writing on international law, including treatises and articles. Such materials are considered "secondary sources," both in a bibliographic sense and in terms of the sources of international law. For a selective list of treatises, and an introduction to resources for finding books and articles on an international law topic, please see our research guide Getting Started on International Law Research. As that guide points out, secondary sources can be the best starting point (along with encyclopedias and dictionaries) for substantive understanding of an unfamiliar area of international law research. This category may also include the work of organizations such as the International Law Commission and private institutions.

For Further Reading

Pratter, Jonathan, "International Law," in Fundamentals of Legal Research, ed. By Steven M. Barkan, Roy M. Mersky and Donald J. Dunn, 9th ed.( Foundation Press, 2009), pages 419-482.
MON KF 240 .B268 2009 (ON RESERVE) and MON KF 240 .B268 2009

"International Legal Research," in Public International Law in a Nutshell, by Thomas Buergenthal and Sean D. Murphy, 4th ed. (Thomson/West, 2007), pages 357-382
I,MON K 3410 .B84 2007 (ON RESERVE)

Borgen, Christopher J., "Introduction: Of Maps, Stories and Tapestries: Researching International and Domestic Law in the Age of Globalization," in Guide to International Legal Research, The George Washington University Law School International Law Review (LexisNexis, 2010), pages 1-33. (I,REF KZ 1234 .G85 2009)
Also available in LEXIS

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