The articles in this volume trace the development of the theory that humanity forms a single world community and that there exists a body of law governing the relations among the members of that community. These ideas first appeared in the writings of the medieval canon lawyers and received their fullest development in the writings of early modern Spanish intellectuals. Conflict and contact with ‘the infidel’ provided a stimulus for the elaboration of these ideas in the later Middle Ages, but major impetus was given by the English subjugation of Ireland, and by the discovery of the Americas. This body of work paved the way for the modern notions of an international legal order and universal norms of behavior usually associated with the publication of Hugo Grotius’s work in the seventeenth century.