Vienna Convention on the Law of Treaties

The Vienna Convention on the Law of Treaties (VCLT) is an international agreement that regulates treaties among sovereign states. Known as the treaty on treaties, it establishes comprehensive, operational guidelines, rules and procedures for how treaties are drafted, defined, amended and interpreted, and it defines a treaty as an international agreement in writing concluded between states governed by international law. The convention was adopted and opened for signature on 23 May 1969, entered into force on 27 January 1980, and had been ratified by 119 sovereign states as of May 2026; non-ratifying states such as the United States have nonetheless recognized parts of it as a restatement of customary international law. Its own definition of a treaty, limited to agreements between states and excluding non-state actors such as international organizations, is not universally accepted.