This chapter provides an in-depth analysis of the role reciprocity plays in treaties. First looking at treaty law and how reciprocity functions in the rules on reservations to treaties, the chapter goes on to examine reciprocity’s role in bilateral treaties, drawing on the examples of air transport agreements and energy agreements in particular, before looking at reciprocity’s role in multilateral treaties of the bilateralizable, interdependent, and integral types. It then analyzes treaties creating international organizations, particularly the EU and ILO, and differentiated obligations in environmental and trade law, illustrating how reciprocity operates in these types of instrument. Finally, the chapter addresses "objective regimes" and the effects of treaties on third parties. While some limitations exist on reciprocity, these do not depend on the substance of obligations but rather on the legal equality of the subjects involved.

International law depends on foreign relations law not only to establish the means by which states can consent to treaty obligations, but also the means to fulfill any resulting obligations, including by incorporating treaties into domestic law. Given that, the professed indifference of international law towards it—as toward other forms of domestic law—is puzzling. It is also misleading. As is widely evident, international affairs create an opportunity for certain domestic actors, usually executives, to assume additional authority. Less obviously, international law, reflected in the Vienna Convention on the Law of Treaties (VCLT), actually influences foreign relations law relating to the creation and elimination of international treaty obligations – encouraging even those states that possess other constitutional agents to regard executive power as sufficient. The homogenizing effects of particular international law rules is an important complement to the study of comparative foreign relations law.

The chapter focuses on the distinction between treaties and other kinds of international instruments. It addresses issues such as why it is necessary to distinguish between treaties and other kinds of international instruments, and what can go wrong when a State or an IO fails to do so. It identifies other kinds of international instruments that are not treaties, their binding status, different scenarios in which it may be difficult to differentiate such instruments and how to overcome such situations. The chapter also offers some suggestions on handling treaties and other kinds of international instruments.