Giles of Rome was the archbishop of Bourges and a loyal champion of Pope Boniface VIII during the Franco-papal crisis of 1296--1303. On Ecclesiastical Power was written at the height of the conflict between Pope Boniface VIII and King Philip IV of France and represents the earliest fully articulated and philosophically developed defense of the "hierocratic" ideology of the medieval papacy. Giles's theory that all government must be legitimized by the pope was based on scriptural, philosophical, patristic, and canonical sources, and his conclusion that the pope is the rightful ruler and final judge of the world -- even in secular matters -- is the definitive statement on papal power in the Middle Ages. This book offers a new and complete critical edition of the Latin text, based on a collation of five fourteenth-century manuscripts. It is accompanied by a literal English translation and a detailed introduction analyzing the context and content of the treatise. The book takes into account the hitherto unconsidered Cremona manuscript of On Ecclesiastical Power and will be indispensable to scholars and students of the history of political thought and international relations.