The Vienna Convention was done at Vienna on 23 May 1969 and entered into force on 27 January 1980. It is regarded as one of the most important instruments in treaty law and remains an authoritative guide in disputes over treaty interpretation.
The Vienna Convention on the Law of Treaties (VCLT) is an international agreement regulating treaties between states. It establishes comprehensive rules, procedures, and guidelines for how treaties are defined, drafted, amended, interpreted, and generally operated.
An international treaty is a written agreement between international law subjects reflecting their consent to the creation, alteration, or termination of their rights and obligations.
The VCLT is considered a codification of customary international law and state practice concerning treaties.
The convention was adopted and opened to signature on 23 May 1969, and it entered into force on 27 January 1980.
It has been ratified by 116 states as of January 2018. Some non-ratifying parties, such as the United States, recognize parts of it as a restatement of customary international law and binding upon them as such.
Vienna Convention on the Law of Treaties_1969