This is a full-scale study of the political thought of the Italian jurist, Baldus de Ubaldis (1327–1400). Baldus shared with his teacher and colleague, Bartolus of Sassoferrato, the greatest fame and influence amongst the Commentators, the school of jurists which dominated Roman law studies in the late Middle Ages and remained highly influential throughout the sixteenth century and beyond. Baldus was also a canonist of renown. Although Baldus was certainly the juristic peer of Bartolus, he has previously attracted far less attention from modern scholars. This book is particularly concerned with Baldus' treatment of universal and territorial sovereignty; his contribution to the development of the idea of the state; his theory of the sovereignty of independent city-republics; his ideas of citizenship; and his discussion of kingship and signorie. Baldus was, in short, a major contributor to the juristic mainstream in European political thought in the late medieval and Renaissance periods.