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", the VCLT establishes comprehensive, operational guidelines, rules, and procedures for how treaties are drafted, defined, amended, and interpreted. The VCLT defines a treaty as an international agreement in writing concluded between states governed by international law.

The Vienna Convention on the Law of Treaties was adopted and opened to signature on 23 May 1969, became effective on 27 January 1980, and has been ratified by 118 sovereign states as of February 2026. Non-ratifying parties, such as the U.S., have recognized parts of the VCLT as a restatement of customary international law. In treaty law, the VCLT is the authority for resolving disputes about the interpretation of a treaty.

The VCLT definition of a treaty is not universally accepted. For instance, VCLT defines a treaty as between states, excluding non-state actors (such as international organizations). Treaties may also be defined differently under domestic law, as certain prerequisites have to be met for something to count as a treaty.