This chapter offers a general definition that seeks to capture what is common to specific and diffuse reciprocity. It analyzes the meaning of reciprocity in the game-theoretic literature and explores the strengths and weaknesses of specific reciprocity as a principle of action designed to elicit mutually beneficial cooperation in world politics. The chapter investigates how specific and diffuse reciprocity are related in practice, and how they could be combined to gain some of the advantages of both. This discussion emphasizes the importance of institutional innovations that can facilitate international cooperation. The literature on reciprocity in international relations emphatically associates reciprocity with equivalence of benefits. Great Britain enacted the Reciprocity of Duties Act in 1823, under the leadership of William Huskisson, the president of the Board of Trade. Specific reciprocity is an appropriate principle of behavior when norms of obligation are weak—the usual case in world politics—but when the occurrence of mutually beneficial cooperation seems possible.