This article, drawing on Marsilius of Padua's Defensor Pacis, discusses Marsilius's theory of dominium, situating that theory within the context of the debate with Pope John XXII and William of Ockham. The author also reintroduces the long unsettled question of the extent of Marsilius's legal knowledge and training. The article closes by calling for a more sustained investigation of Marsilius's knowledge of Roman law, and of his relation to the poverty controversy and especially Ockham.