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Public International Law: Customary Law

This guide highlights key resources for international law research.

Starting Points

Article 38(b) of the Statute on the International Court of Justice identifies customary international law as a source of law.  This Globalex article on CIL is a good primer to review sources and approaches to research CIL.  On this page, you will find resources to demonstrate consistent practice by the United States, other nation states, and nonstate actors.



Article 38(1)(b) of the statute of the International Court of Justice identifies “international custom, as evidence of a general practice accepted by law” to be a source of international law.

The Restatement (Third) on the Foreign Relations of the United States provides a good explanation of what is meant by this provision that encapsulates the overall concept as expressed by scholars and experts in the field.

Section 102 and its accompanying notes explain that the custom must result from a general and consistent practice of states that is followed out of a sense of legal obligation (opinion juris sive necessitates). This is generally interpreted to meant that there is:

  • widespread repetition of the practice
  • by a significant number of states (the corollary to this means that the practice cannot be rejected by a significant number of states if it is to be considered custom under international law)
  • And has not be rejected by a substantial number of states

The sources used to substantiate a custom are broad and result from careful study.  Custom can be evidenced by treaties, resolutions and declarations of intergovernment organizations, diplomatic materials, and other official statements from sovereign nations including legislation and case opinions.  Several categories and resources are identified on this page.  See other tabs in this guide for related material on finding international case law, treaties and international organizations.

Nonstate Actor Sources of Customary Practices

The most significant nonstate actor is the United Nations but you will also want to check the practices of other intergovernmental organizations (See appropriate tabs in this guide for intergovernmental organizations and the United Nations specifically).  

Some quick links to United Nations Sources are:

You can also use PAIS subscription resources to help research country positions on international issues:

  • PAIS (Public Affairs Information Service) - PAIS International is an index to the world's public and social policy literature referencing and abstracting materials from journals, books, gray literature, internet material, conference proceedings, government and ngo documents and statistical publications. Coverage starts in 1915. This is not a full text database.



United States Sources

The Restatement on Foreign Relations is an important, authoritative source on U.S. foreign relations law that can aid in understanding this area of law.  In addition to the compilations below, consult Sean D. Murphy's United States Practice in International Law (2002-2004)  for additional background material in this area.

Digests of International Law - These resources digest the activities of the three branches of government as they relate to foreign and international affairs.

Diplomatic Materials - The Department of State Bulletin was the official record of U.S. foreign policy from 1939-1989. It was replaced the Dispatch.  The Dispatch was discontinued at the end of 1999. Current briefings and press releases can be found on the Department of State public affairs web site.

Resources for older materials are listed below:

Non-U.S. State Sources

Yearbooks, International Tribunals and Digests of International Law - There are no true digests of international law for other countries, nor are there equivalents to the Restatement on Foreign Relations.  The closest type of resource would be yearbooks on international law from other countries, or general yearbooks on an international law topic or foreign jurisdiction. The HeinOnline Foreign & International Law Library includes international yearbooks, serials, decisions of international tribunals and digests of international law.

You can also search PRIMO using the word "yearbook" in the title and a keyword representing the jurisdiction of topic of choice.

Diplomatic Materials - Like the United States, other countries reproduce documentation regarding foreign affairs.  You should look to each country's department or ministry that is equivalent to the U.S. Department of State for documentation.  Web sites for these departments can be easily found by using the Department of State World Wide Diplomatic Archives Index which can also lead you to other diplomatic materials for foreign jurisdictions.