This study addresses what has been a previously unexamined subject in Luther scholarship and presents how Luther (re-)interprets the concept of purgatory in line with his biblical and theological insight. The book gives the background of the development of the doctrine of purgatory, particularly of penitential theology, as well as a brief summary of Luther's so-called Reformation breakthrough. Furthermore, it explicates how in the Explanations of the Ninety-Five Theses (1518) Luther articulates the notion of purgatory within the framework of justification by faith alone and the theology of the cross. So this book demonstrates the way a minor subject in Luther--purgatory--interacts with his major theological subjects--justification and the theology of the cross. Careful analysis helps readers follow Luther's way of thinking in the course of the Reformation.