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International and Foreign Law Research: Customary Law & General Principles

This guide is intended to assist students in the International & Foreign Legal Research class at Florida State University locate important resources.

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!

Evidence of State Practice

Even though customary practices are not usually written down, international lawyers will look to written evidence of state practice documented in a variety of sources. The International Law Commission, the Restatement on Foreign Relations, and many "highly qualified publicists" give us some guidance as to where to look for this written evidence. 
  • Diplomatic Practice
  • Domestic Legislation   
  • Practice of Intergovernmental Organizations 
  • International Decisions
  • Opinions of legal advisors
  • Official military manuals
  • Policy statements

What are Generally-recognized Principles of Law?

The "general principles of law recognized by civilized nations" is the third source of law according to the Statute of the International Court of Justice. General principles are legal concepts so fundamental that they can be found in legal systems throughout the world. In researching general principles of international law, consult background sources, international decisions, and domestic legislation. For a more nuanced discussion, see International Judicial Monitor, What are General Principles of International Law?" July/August 2007. 
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