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Foreign, Comparative & International Law

What is Customary International Law?

Customary International Law

“Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation.”

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.”

When is state practice considered to be customary international law? The Restatement requires both

  • a general and consistent practice and
  • a sense of legal obligation (opinion juris)

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.”Obviously, 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.” From GlobalexResearching Customary International Law, State Practice and the Pronouncements of States regarding International Law by Silke Sahl


Restatement (Third) of Foreign Relations Law § 103. Evidence Of International Law> (1987)

(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.

Jus Cogens

Jus Cogens: Black's Law Dictionary Deluxe Ninth Edition

  • International Law. A mandatory or peremptory norm of general international law accepted and recognized by the international community as a norm from which no derogation is permitted. A peremptory norm can be modified only by a later norm that has the same character.
  • Civil Law. A mandatory rule of law that is not subject to the disposition of the parties, such as an absolute limitation on the legal capacity of minors below a certain age. Also termed (in sense 2) peremptory norm 

Picture of Judge A.A. Cancado Trindade

Video Link (40 minutes)

Source:  United Nations Audio Visual Library, Customary International Law: Jus Cogens

Judge A. A. Cançado Trindade, International Court of Justice

Codification Division, Office of Legal Affairs
Copyright © United Nations, 2017. All Rights Reserved.
Terms and Conditions of Use.

Research Strategies

Customary Law Can Be Difficult to Research:  Strategies for Success

  • Start With Secondary Sources (Encyclopedias & Dictionaries)
  • Look for Writings of Publicists (Commentary & Monographs)
  • Then, International Law Decisions
  • Treaties & Agreements
  • State Practice
    • Decisions of National Courts
    • Opinions of National Legal Advisors
    • Diplomatic Correspondence
    • Records of a Nation's Foreign Relations & Diplomatic Practices
    • Presidential Papers
    • Papers of Departments of Defense, State, National Security Council, CIA, Agency for International Development (U.S.) See:
  • Practice of International Governmental Organizations (IGO) 
  • National Laws
  • Factual Research 

State Practice - U.S.

U.S. Practice

Documenting state practice is important to showing evidence of international law.

Besides searching digests and yearbooks, Foreign Relations of the United States is a primary way of searching for practices of the United States.  Also consider the Restatement of the Law, Third, Foreign Relations.

Catalog Subject Searches - Examples

Catalog Subject Searches - Examples

Yearbooks and digests may have demonstrate international and state practice and thus constitute evidence of international law.

Digests and yearbooks are a primary way of discerning the practice of states.

Treatises - Online and in Print

Treatises - Online and in Print

Treatises can constitute the evidence of publicists on international law.  Which treatises are such evidence is open to interpretation.

Web Guides & Tutorials

Web Guides and Tutorials

Top Online Resources

Top Online Resources

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