This unique study makes both a substantial contribution to our understanding of Roman political thought and a major contribution to the reception of Roman ideas about politics. The book reorients the discussion of the debt of early modern political thought from the familiar claims about republicanism and republican virtue to the rediscovery of a tradition of Roman constitutionalism. In the first part, we learn how a Roman concept of constitution emerged out of the crisis of the Republic. The emergency powers of the late Republic provoked Cicero and other contemporaries to turn an inchoate constitutionalism into explicit constitutional argument and constitutional theory. The crisis of the Republic thus brought about a powerful constitutionalism and convinced Cicero to articulate the norms and rights that would provide its substance; this typically Roman constitutional theory is described in the second part. Part three discusses the reception of Roman constitutional thought up to the late eighteenth century and the American Founding, which gave rise to a new constitutional republicanism. Special attention is paid to Jean Bodin, who emerges as a key thinker in a tradition leading up to Montesquieu and, eventually, the Federalist and John Adams. This tradition was characterized by a keen interest in the Roman Republic’s decline and fall and an insistence on the limits of virtue. The crisis of the Republic was interpreted as a constitutional crisis, and the only remedy to escape the Republic’s fate—military despotism—was thought to lie, not in republican virtue, but in Roman constitutionalism.