Customary international law refers to obligations that arise from established state practice rather than from formal written agreements, such as treaties. Customary law on state responsibility for wrongful acts, including the expropriation of private property, is highly relevant to the study of international investment law.
Articles on Responsibility of States for Internationally Wrongful Acts.
These articles, which were adopted by the International Law Commission of the United Nations in 2001 after almost four decades of discussion, codified the customary law on state responsibility. For a detailed analysis of this codification, consult the reference book described below.