It is often said that reciprocityÑreturning good behaviour for good behaviour and bad for badÑstabilizes co-operation by making nonco-operative behaviour unprofitable. Nevertheless, our understanding of how reciprocity works in international politics is unnecessarily limited. Only two patterns are now well recognized. In one of these, both actors in a relationship insist that the value of their concessions must be equivalent and that each must be made highly conditional on the other. The polar opposite pattern is one in which the actors consider both the value and timing of individual concessions to be irrelevant. What is not well understood is that there are two other exchange patterns that actors may also see as mutually satisfactory. In these situations, even though the value and timing of individual concessions both clearly matter to the actors, the pattern of exchange on oneÑthough not bothÑof them is clearly ‘unbalanced’. We lack a way to make sense of such partially unbalanced relationships. This article develops a typology that specifies four types of reciprocity, analyses the conditions under which they are likely to evolve, and draws out the implications of each pattern for actors' strategic choices and bargaining behaviour. The utility of this typology is demonstrated through an analysis of the shifting patterns of reciprocity in Russian-American relations from 1947 through to 1997.