Treaties are considered the most important source of international law. The Vienna Convention on the Law of Treaties provides in Article 2 that "'treaty' means an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation."

Though international organizations and other actors recognized under international law also may conclude treaties, states are generally the main contracting parties to treaties. Treaties create legal rights and obligations between the contracting parties and therefore are legally binding upon them. Treaties can have a variety of names, including treaty, agreement, charter, convention, protocol and exchange of notes.

Whether a document is a treaty (which means that it is legally binding) must be decided by taking into account the terms, context and contracting parties, as well as other factors which demonstrate the intention of the parties. Before concluding a document, the contracting parties must agree on its character, including whether it is a treaty.