The first two pages of the Treaty of Brest-Litovsk, in (left to right) German, Hungarian, Bulgarian, Ottoman Turkish and Russian

A treaty is a formal, legally binding written agreement between actors in international law. It is usually entered into by sovereign states and international organizations,[1] but can sometimes include individuals, business entities, and other Legal persons.[2] A treaty may also be known as an international agreement, protocol, covenant, convention, pact, or exchange of letters, among other terms. Regardless of terminology, only instruments that are legally binding upon the parties are considered treaties pursuant to, and governed by, international law.[3]

Treaties are roughly analogous to contracts, in that they establish the rights, duties, and binding obligations of the parties.[4][5] They vary significantly in form, substance, and complexity, and may govern a wide variety of matters, such as territorial boundaries, trade and commerce, mutual defense, and more. Treaties establishing international institutions often serve as the constitution thereof, such as the Rome Statute of the International Criminal Court and the Charter of the United Nations.

Treaties are among the earliest manifestations of international relations, with the earliest known example dating to the mid-13th century BC, and are today recognized as a primary source of international law.[6] International law on treaties have mostly been codified by the Vienna Convention on the Law of Treaties, which sets forth the rules and procedures for creating, amending, and interpreting treaties, as well as for resolving disputes and alleged breaches.[7] However, most treaties have internal procedures and mechanisms governing potential disagreements.