Start Your Research: (2) Custom

A basic introduction to international legal research in the Peace Palace Library. Your road map for approaching an international legal research problem. Step 2.

Article 38(1)(b) of the Statute of the International Court of Justice states that the Court shall apply 'international custom, as evidence of a general practice accepted as law' to settle international disputes. A rule of customary international law is demonstrated by showing evidence of two essential elements: 'general practice' and 'accepted as law'. The first element is objective and the second is subjective, referred to as opinio juris sive necessitates, or simply, opinio juris.

Customary international law develops in a more informal and decentralised manner than treaty law. It arises organically, based on the activities and beliefs of States acting on the international stage over time. It is unwritten and contains no formal enforcement mechanisms whatsoever. In the context of legal research, one must be focused on the resources that reveal patterns of behaviour as States pursue their national interests, and on their respective motivations underlying their patterns of behaviour. Once a rule of customary international law emerges, it binds all States, whether or not they participated in its development.

There is no single, clear path for finding and determining rules of customary international law because the documents vary depending on the subject matter of the custom. Using publications and documents from multiple foreign jurisdictions is particularly challenging. Taking the time to understand customary international law as a source of law before starting the research process is essential. By following the strategies discussed within the reference works suggested below, legal researchers should be able to handle the research challenges posed by customary international law as a source of law. Please, consult our Research Guide on Customary International Law as a starting point for finding the right resources.

Step 1 ← Step 2 → Step 3

You might consider our Library Instruction on how to conduct legal research in the Library at all levels.