Convinced that the antichrist was “at our gates,” the Franciscan friar, alchemist, and prophet John of Rupescissa (ca. 1310-1366) wrote the Book of Light of the Great Magistery to help his future brethren restore a devastated Christendom. Written in 1354, this text provides a detailed process for making the philosophers’ stone, the substance able to transmute base metals easily into gold and silver. With this manufactured gold and silver, the true Franciscans, the “poor men of the Gospel,” could rebuild the Church and help usher in an era of peace and prosperity. This volume presents the first critical edition, modern English translation, and focussed study of John’s Book of Light. Based on over one hundred surviving manuscripts, many recently discovered, this critical edition restores substantial original text that was omitted from the printed editions and corrects longstanding textual errors. In the accompanying study, Principe explores John’s sources and ideas, especially his striking theories of matter and material change, and follows the transmission, reception, vernacularization, and multiple modifications of John’s text as readers engaged with it over the next three centuries. Modern laboratory reworkings, fully illustrated and explained, go hand-in-hand with the textual analyses, providing a vivid picture of the friar’s practices and observational skills and deeper understanding of his text. John of Rupescissa stands revealed as an innovative theorist and observant practitioner; he was not an “armchair” alchemist whose knowledge came only from textual sources. The clear evidence of his practice presented here provokes in turn a close reevaluation of the conditions of his nearly twenty-year confinement at a papal prison in Avignon where he learned and practiced much of his alchemy before writing the Book of Light. Historians of science and technology, medievalists, book historians, manuscript scholars, and many others will find new and important information in this third volume of the series Sources of Alchemy and Chemistry.