There is a common perception of reciprocity as a concept that is opposed to the communitarian interests that characterise contemporary international law. Reciprocity is instead seen by many authors as a concept belonging to a bygone era of classical international law, or merely a way of denoting reactions to unfriendly or wrongful conduct. The present thesis instead disputes this approach, and hypothesises that reciprocity is instead linked to the structural characteristic of sovereign equality of States in international law. To conduct this examination, this thesis first carries out an in-depth analysis of the concept of reciprocity, looking outside international law, indeed outside law itself, to understand the nature of the concept and define reciprocity and the elements that characterise it. The second part of the thesis then examines the various roles reciprocity may play and its various articulations in a number of fields of public international law: the law of treaties, the treatment of individuals, the execution of international law, and the jurisdiction of international courts and tribunals. In all these areas, both more traditional and more contemporary examples are analysed, in order to demonstrate that the persistence of reciprocity in public international law is closely linked to the latter’s horizontal structure, based on the sovereign equality of States.