This chapter analyzes reciprocity in the social sciences, historical forms of law, and domestic contexts, including the law of contracts and in federal States, to shed light upon some of reciprocity’s fundamental characteristics. Reciprocity is not incompatible with the existence of a community, but necessarily requires a social relation, and one of its defining characteristics is its relationship with equality. Rather than being a negative concept, based on occasional and discontinued instances of interaction on the basis of reactions to conduct, reciprocity is a concept fundamental to the existence of social relations, and inherent to ideas of justice and fairness.