In international relations, reciprocity describes an environment in which States support one another for short- or long-term advantage through the balancing of rights, duties and interests. This book examines reciprocity in the context of international law. It considers the role reciprocity plays in the creation and development of international law as well as in the interpretation and application of international law. The book illuminates the reciprocal framework of international law and international relations by examining the role reciprocity plays in different types of States’ obligations, including bilateral, bilateralisable multilateral, non-bilateralisable multilateral and obligations erga omnes. The book examines how reciprocity is intertwined with the principle of equality, as the rights and obligations of States are equal irrespective of size and economic or military strength, and the beneficial effects of reciprocity in creating stability and cooperation amongst States.