The Vienna Convention on Treaty Law (VCLT) is an international agreement regulating treaties between states.  The contract, known as the “contract,” contains detailed rules, procedures and guidelines for the definition, development, modification, interpretation and general operation of contracts.  The VCLT is considered to be the codification of customary international law and state contract practice.  However, agreements between states and international organizations or between international organizations themselves are governed by the 1986 Vienna Convention on treaty law between states and international organizations or between international organizations when it enters into force. Moreover, in treaties between states and international organizations, the provisions of the Convention continue to apply between states.  The convention does not apply to unwritten agreements.  The Convention codifies several foundations of contemporary international law. It defines a treaty as “an international agreement that is written and governed by international law between states” and asserts that “every state has the capacity to conclude treaties.” Article 1 limits the application of the convention to written contracts between states, with the exception of treaties between states and international organizations or international organizations themselves. Article 26 defines pacta sunt servanda, article 53 proclaims the mandatory standard, and Article 62 proclaims a fundamental change in circumstances. The agreement applies only to contracts concluded after its creation and to contracts between states and therefore does not regulate agreements between states and international organizations or between international organisations themselves, but if one of their rules is binding on these organisations, they will remain so.  The VCLT applies to interstate contracts within an intergovernmental organization.  The agreement has been referred to as a “contract treaty” and is widely recognized as a relevant guide to the training and impact of contracts.
Even countries that have not ratified it recognize its importance. Thus, the United States recognizes that certain parts of the convention constitute a binding customary right for all nations.  In India, the Supreme Court also recognized the usual status of the Convention.  This Convention may be signed by all States Parties to the United Nations or to one of the specialized institutions or the International Atomic Energy Agency or parties to the status of the International Court of Justice, as well as by any other State that is invited by the United Nations General Assembly to become a party to the Convention. , as follows: until 30 November 1969 at the Federal Ministry of Foreign Affairs of the Republic of Austria, and until 30 April 1970 at the United Nations headquarters in New York. The Convention was adopted on 23 May 1969 and opened for signature and came into force on 27 January 1980.  It has been ratified by 116 states since January 2018.  Some parties that do not ratify, such as the United States, recognize parts of it as a redefinition of customary law and are binding on them as such.  The act of signing and ratifying a treaty as a negotiating state has the same effect as the act of accession to a treaty (or “accession of a treaty”) by a state that has not participated in its negotiations.  Membership normally does not take place until after the treaty enters into force, but the SECRETARy-general of the United Nations has sometimes accepted accessions even before a treaty comes into force.  The only drawback of not being a negotiating state is that there is no influence on the content of a treaty, but there are still reservations about certain provisions of the Treaty to which one wishes to adhere (Article 19).