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International Moot Court Guide: Customary International Law

What is customary international law?

According to Article 38 of the Statute of the International Court of Justice, the second most significant source of international law is international custom. Specifically the ICJ statute states that the court shall apply international custom as “evidence of a general practice accepted as law”. An international lawyer must prove two basic elements for a particular norm to qualify as customary law:

  • State practice must be generally consistent

  • Practice must occur out of a sense of legal obligation.  

Secondary Sources STATING Custom


Primary Sources STATING Custom

Virtually all states have signed the following conventions. 

General Assembly resolutions, in and of themselves, do not state rules of international law.  But, they can result in creating customary international law. Use the following sources to locate resolutions.

Best known resolutions that are in the form of an international agreement are:

Examples of UN General Assembly resolutions that are viewed as customary law are:

To locate judicial opinions that contain a statement that there is a a rule of cutomary international law in effect, consult a well-respected treatise that confirms the statement or consult a database of international opinions.


  • General international law treatises listed here
  • International law treatises on specialized topics. 



When using the databases above, possible search terms and strategies include the following:

  • [rule] AND principle AND custom!
  • [rule] /p principle /p custom!
  • [rule] /s principle /s custom!

Sources of Evidence of State Practice & Opinio Juris

Diplomatic Papers of the United States
Diplomatic Papers of Other Countries

To access compiled collections of diplomatic records of other countries, you may also want to search your online catalog using a subject search of:  

  • [country name] AND foreign relations
  • [country name] AND history AND sources

You may also want to consult the following sources below for determining what titles exist for a particular country other than the United States. 


For a comprehensive guide on finding foreign legislation, consult THIS RESEARCH GUIDE.

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