Treaties are made between subjects of international law, in particular between states, between states and international organisations and between international organisations. This book deals primarily with treaties between states, though Chapter 22 deals with treaties with and between international organisations. Every state possesses the capacity to conclude treaties (Article 6). This chapter is concerned with how the treaty-making power is exercised by a state; by parts of a state (such as the constituent units of a federal state); or by, or on behalf of, overseas territories.

A treaty made between states may be expressed to be made by heads of state, or on behalf of the states, their governments or, less often, their ministries or state agencies. A head of state treaty is used when the subject matter is of exceptional political importance, such as the various constituent treaties of the European Community, though it is not necessary for heads of state to sign in person, and constitutional monarchs do not usually sign anyway. There is no difference in international law between a treaty concluded on behalf of states and one concluded on behalf of governments, ministries or state agencies, since a treaty entered into by a government, ministry or agency binds the state, and a change of government will, in itself, not affect its binding force.