There is a common perception of reciprocity as a concept that is opposed to the communitarian interests that characterise contemporary international law, or merely a way of denoting reactions to unfriendly or wrongful conduct. This book disputes this approach, and highlights how reciprocity is instead linked to the structural characteristic of sovereign equality of States in international law. This book carries out an in-depth analysis of the concept of reciprocity and the elements that characterise it, before examining the various roles and articulations of reciprocity 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, it analyses both more traditional and more contemporary examples, to demonstrate how reciprocity is closely linked to the very structure of public international law.