This study is the first comprehensive treatment of the way Marsilius of Padua (1270/1290–1342), a seminal political thinker of the Late Middle Ages, elaborated on Aristotle’s political thought in articulating his political theory. Its main thesis is that Marsilius is committed to the view of a sharp disjunction between ethics and politics, thus deviating radically not only from Aristotle, but also from the majority of medieval Commentators of Aristotle such as Thomas Aquinas, Giles of Rome, Peter of Auvergne and John of Jandun. From a methodological viewpoint, it follows the model of “concentric circles”: its starting point is a detailed comparison of the main ideas of the Defensor pacis (Defender of Peace) (1324), Marsilius’ opus magnum, on the one hand, and Marsilius' quotations from William of Moerbeke’s Latin translations of Aristotle’s works focusing in particular on the Politics, the Nicomachean Ethics, the Rhetoric and the Metaphysics as well as the Greek original, on the other. This method of exploring the use of classics in medieval political thought challenges the notion of “reception” as conceived of by recent research and brings out a number of crucial differences between Marsilius and Aristotle. Further, this study situates Marsilius’ ideas in the history of late medieval political thought (podestà literature etc.), the political realities that obtained in 14th-century Padua and late medieval debates on the fortunes of the communal regime and on the rise of the signoria (e.g. Albertino Mussato). It also argues against Marsilius’ so-called Averroism and identifies Moses Maimonides’ Guide of the Perplexed as a potential source of the Defensor pacis.