While the works of Hugo Grotius (1583-1645) have long been held in high esteem by international lawyers, this book addresses the broader, and neglected, theme of his contribution to the theoretical and practical aspects of international relations. It critically reappraises Grotius' thought, examining it in relation to his predecessors and in the context of the wars and controversies of his time, and assesses the strengths and weaknesses of the `Grotian' tradition of thought - one which accepts the sovereignty of states but at the same time stresses the existence of shared values and the necessity of rules.