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.