Marsilius of Padua is conventionally considered to be ahead of his time as the first secular political theorist, the first post-classical thinker to espouse republicanism, and a scholastic precursor of the civic humanists of the renaissance. This book attempts to overturn this view, by advancing the first historical interpretation of Marsilius's thought. It examines the neglected evidence for Marsilius's life, and for contemporary responses to his best-known work, Defensor Pacis. Particular attention is given to the second discourse of the Defensor, which tends to receive short shrift in modern scholarly discussions; detailed comparison is also made with Marsilius's lesser-known works. The book argues that Marsilius was not a republican, but an imperialist, and a loyal servant of Ludwig IV, rex Romanorum and claimant to the imperial title. Far from being a precocious work of secular political theory, the Defensor Pacis is an anti-papal polemic underpinned by a profound Christian understanding of history as a providentially ordained process. In this process Marsilius attributes great significance to the conversion of the Emperor Constantine, as the point when the church founded by Christ and the Roman Empire began to coalesce.