This book presents an argument about the foundations of Hugo Grotius’ system of natural law. While Grotius provided a very wide variety of citations to develop and bolster his theory, only certain kinds of sources were authoritative, namely Roman law and Ciceronian ethics – or so this monograph argues. My book does not simply take Grotius’ practice of citation as conclusive, but instead offers an argument that specific Roman sources were much more important to Grotius than the other texts he cites and indeed provided the foundations for his highly influential system of natural jurisprudence. Parts of this book have appeared previously in various places, and I am grateful to the editors and publishers of that material for allowing republication in the present book: “Appetitus societatis and oikeiosis: Hugo Grotius’ Ciceronian Argument for Natural Law and Just War,” Grotiana 24–25 (2003–4); “The Right to Punish as a Just Cause of War in Hugo Grotius’ Natural Law,” Studies in the History of Ethics 2 (February 2006); “‘Ancient Caesarian Lawyers’ in a State of Nature: Roman Tradition and Natural Rights in Hugo Grotius’ De iure praedae,” Political Theory 34, no. 3 (2006); “Natural Rights and Roman Law in Hugo Grotius’s Theses LVI, De iure praedae and Defensio capitis quinti maris liberi,” Grotiana 26–28 (2005–7), also published in H. W. Blom, ed., Property, Piracy and Punishment: Hugo Grotius on War and Booty in De iure praedae (Leiden: Brill, 2009); “Is Modern Liberty Ancient? Roman Remedies and Natural Rights in Hugo Grotius’ Early Works on Natural Law,” Law and History Review 27, 1 (Spring 2009); “Introduction” (with Benedict Kingsbury), in AlbericoGentili, TheWars of the Romans: A Critical Edition and Translation ofDe armis Romanis, ed. B. Kingsbury and B. Straumann (Oxford: Oxford University Press, 2011).