The Vienna Convention on the Law of Treaties (VCLT) (1969) is the key point of reference. The VCLT reflects, to a very large degree, customary international law.

The VCLT regulates treaties concluded between States. It was followed by the 1986 Vienna Convention on the Law of Treaties between States and International Organizations or between Organizations, which is not yet in force.

Treaties are one of the means through which States regulate their international relations. They also constitute one of the formal sources of international law according to Art 38(1) International Court of Justice (ICJ) Statute.

The cornerstone of the law of treaties is the principle pacta sunt servanda, namely what has been agreed to must be respected.