Customary International Law, General Principles of Law, and Other Sources

Research Guides & Background Information

Background Information

Department of State

Research Guides & Background Information


Domestic Law Concerning a Country's International Obligations

Resolutions, Declarations, and Legislative Acts of Intergovernmental

IGO yearbooks


Judicial Decisions of
International Tribunals

Court Decisions of Individual Countries Concerning International Obligations

Scholarly Works

Review Questions

Customary International Law: Research Guides & Background Information

Introduction: General Principles of Law and Other Sources

Article 38 of the International Court of Justice Statute lists customary international law and general principles of law as two of the sources of international law. These are the two most difficult sources to research because they are documented in such a wide variety of materials (e.g., state papers, diplomatic correspondence, executive decisions, judicial decisions, etc.).

Article 38 also lists judicial decisions and the teachings of international law scholars as “subsidiary means for the determination of rules of law.” These are not authorities, rather they are evidence of the sources of international law.

Research Tip
Because the sources of customary international law and the general principles of law are so diverse, secondary sources are useful places to start your research.