This chapter expands on the concept of reciprocity, looking at how it can be defined, its key characteristics, and the three key functions it plays: in norm creation, as a condition, and in execution of the law. It examines the defining elements of reciprocity: proportionality, relativity, equality, and interdependence. The chapter goes on to examine the nature of reciprocity, and its relationship to custom and general principles, two sources of international law. It illustrates how it is reciprocity’s relationship with the structural characteristic of sovereign equality in international law that explains many of its roles and defining characteristics.