Customary international law

Customary international law refers to international obligations arising from established state practice, as opposed to obligations arising from formal written international treaties.  According to Article 38(1)(b) of the ICJ Statute, customary international law is one of the sources of international law.  Customary international law can be established by showing (1) state practice and (2) opinio juris. 

See also:

• Sources of international law
• International conventions