The Italian Dominican friar Silvestro Mazzolini da Prierio (1456–1527), known as Prierias, is chiefly remembered as the church official designated to respond to Luther’s 95 theses of December 1517-a response blamed for fanning the flames of the Reformation throughout Europe. In Prierias, Michael Tavuzzi presents the first full biography of this little-known, yet eminent, sixteenth-century ecclesiastic, as well as an account of his wide-ranging literary works. Tavuzzi shows that, aside from being Luther’s first opponent, Prierias played a key role in significant early-sixteenth-century controversies such as the cases of Reuchlin, Erasmus, and Pomponazzi. Drawing on previously untapped primary sources, Tavuzzi traces Prierias’s early career as preacher, professor, inquisitor, and administrator, and places him in the context of the struggles fought between unreformed and observant friars during the Renaissance. Prierias’s activities as Master of the Sacred Palace, his long-standing and bitter conflict with Cardinal Cajetan, and especially his forceful writings warning against the threat of witches and witchcraft are also described. Focusing closely on Prierias’s clash with Luther, Tavuzzi accounts for its development and illuminates the ecclesiological issues at stake. Prierias reveals the little-known world of the friars on the eve of the Reformation.