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Introduction to International Law: Customary International Law

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What is Customary International Law?

According to the Restatement of the Law Third, Foreign Relations of the United State, Sec. 102(2) (1987), customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation."  

Additionally, the Statute of the International Court of Justice identifies sources of international law in Article 38 as:

  • International conventions (Treaties)
  • International Custom, as evidence of a general practice accepted as law
  • General principles of law recognized by civilized nations
  • 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 (Scholarly works)

Where can I find Customary International Law?

Evidence of customary rules of international law may be found in treaties, national law, international and national jurisprudence, and scholarly writings. Other sources include the practices of individual states, widely ratified treaties as evidence of the customs and practices of states, and long-established trade practices.

Resources for Customary International Law include: 

  • ICRC Customary International Humanitarian Law database has rules of customary IHL and national practice for the rules of customary IHL.
  • Restatement of the Law, Third, Foreign Relations Law of the United States discusses US practice and policy on foreign relations law and includes statements on customary international law. Available on Range 301 in the MacMillan Law Library and online through Hein Online ALI Library, Westlaw, and Lexis.
  • Sources of State Practice in International Law, 2d ed, KZ 64 .S67 2014. Chapters by country describing treaty practice and listing treaty collections and sources.
  • Treatises and journal articles on international law include citable statements of custom and practice, especially international law classics and noted authors:

Research Guides for finding Customary International Law include:

Subject Guide