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Public International Law: Other Sources

Guidance for researching subjects related to public international law.
As discussed in the introduction, article 38 also references three sources of law that are less familiar: international custom, general principles of law, and writings of publicists. These are all areas that can be difficult to define and have a lot of overlap in research sources. 


Customary international law is derived from practices of states and is treated by states as a legal obligation. This is commonly referred to as state practice and opinio juris.

  • State practice looks at what states actually do and what they say they do. 
  • Opinio juris is a principle that means a state believes a certain standard is required by international law. The full term is opinio juris sive neccessitatis and is defined by Black's Law Dictionary as "[t]he principle that for conduct or a practice to become a rule of customary international law, it must be shown that countries believe that international law (rather than moral obligation) mandates the conduct or practice." 

This is a wide definition, and your research can take you through a variety of resources. You could potentially look for national laws, regulations, white papers, and similar government documents in addition to secondary sources. 

Now compare this with general principles of law which is generally discussed as being derived from state practice. You can see how this creates much overlap with the research resources relating to customary international law. 

Writings of publicists are authoritative writings by well-regarded scholars. This is far from being a definite definition. Who is well-regarded and what is authoritative? It can be a "you know it when you see it" area. It is safe to stick to resources from expert and scholarly organizations, such as the American Law Institute and the Restatements they produce.

Digests and Yearbooks

We have many of these resources in print; to browse the collection, search “digest” or “yearbook” on the online catalog, or you can search for a particular title. There are also a wide variety of digests and yearbooks online. 

  • Digest of United States Practice in International Law
  • International Community Law Review
  • International Court of Justice Yearbook
  • International Labor Law Reports
  • International Organizations and the Law of the Sea: Documentary Yearbook
  • International Tribunal for the law of the Sea Yearbook
  • Scandinavian Institute of Maritime Law Yearbook
  • Max Planck Yearbook of United Nations Law
  • Ocean Yearbook
  • Revue Hellenique de Droit International
  • United Nations Disarmament Yearbook
  • United Nations Juridical Yearbook
  • United Nations Yearbook of the International Law Commission
  • Yearbook of the League of Nations


Compilations of State Practice

Scholarly Orgs


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