This book offers the first comprehensive account and re-appraisal of the formative phase of what is often termed the 'Grotian tradition' in international relations theory: the view that sovereign states are not free to act at will, but are akin to members of a society, bound by its norms. It examines the period from the later fifteenth to the mid-seventeenth centuries, focusing on four thinkers: Erasmus, Vitoria, Gentili and Grotius himself, and is structured by the author's concept of international society. Erasmus' views on international relations have been entirely neglected, but underlying his work is a consistent image of international society. The theologian Francisco de Vitoria concerns himself with its normative principles, the lawyer Alberico Gentili - unexpectedly, the central figure in the narrative - with its extensive practical applications. Grotius, however, does not re-affirm the concept, but wavers at crucial points. This book suggests that the Grotian tradition is a misnomer.