The English Franciscan, William of Ockham (c. 1285-1347), was one of the most influential philosophers and theologians in late medieval Europe. Recent scholarship has shown his profound impact on logic, metaphysics, epistemology and the philosophy of language in the late Middle Ages and beyond. Following a dispute between the papacy and his Order, Ockham abandoned his academic career and devoted himself to anti-papal polemics. Twentieth-century scholars have produced divergent and often contradictory interpretations of Ockham as a political thinker: a destructive critic of the medieval Church, a medieval Catholic traditionalist, the Franciscan ideologue, and a constitutional liberal. This book offers a fresh reappraisal of Ockham's political thought by approaching his anti-papal writings as a series of polemical responses. His aggressive and persistent attack on the papacy emerges in this study as an attempt to rescue the ethical foundations of the Christian society from the political influences of heretical popes.