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Guide to International and Foreign Law Research

Important Terms

Both customary international law and general principles of law are defined in Article 38 of the International Court of Justice Statutes as sources of law.

Customary International Law is a body of legal rules that restrict the activities of states and are not written down or codified in any particular source.  Customary international law arises when a significant number of states consistently engage in a pattern of behavior AND the conviction has developed among states that this behavior is required by international law (otherwise known as opinio juris).

State practice is generally determined by two factors: (1) the number of states that behave in accordance with the practice; and (2) the length of time over which those states have behaved in accordance with the practice.

Opinio juris is the general believe that the observed state practice is legally obligatory.

General Principles of Law are legal ideas that are so fundamental that they transcend all national borders (for example, the idea that every legal wrong must occasion a legal means of redressing that wrong).

Researching Customary Law

Researching customary international law is perhaps the most laborious type of international law research.  The most rigorous approach would be finding real world evidence of state practice, and then separately finding evidence of opinio juris supporting that practice.  Most frequently, however, research consult secondary sources, which may argue that certain customary rules exist.  The best secondary sources will contain detailed information about the primary sources from which the rules were derived, allowing the researcher to work backwards to build a case that a certain rule of customary law in fact exists.

The following resources are some of the helpful starting places when formulating research on customary law:

 Max Planck Encyclopedia of Public International Law: Customary Law

This online encyclopedia has a very detailed article and research guide on understanding and researching customary law. Max Planck is available through the law school. You can access it at the law school directly by clicking the link above on any law school computer or from home by logging into the Law Library's electronic resources.


Customary International Law and Treaties (2d ed.) by Mark Eugen Villiger: KZ1277.V55 1997

This book provides an in-depth explanation of the origins of customary law, as well as a modern discussion of the ways in which the different sources of international law interact.


Customary International Law: A New Theory with Practical Implications by Brian D. Lepard: KZ1277.L47 2010

This book takes an interdisciplinary approach to the origins and issues associated with determining customary international law. It delves into the interplay between politics, diplomacy, and ethics in determining state practice and opinio juris.


Practice and Methods of International Law by Shabtai Rosenne: KZ3405.R66 A377 1984

Chapter III of this book provides great insight into the sources of customary international law and the distinctions between state practice and official conventions.