Presents the first full-scale study of the Tractatus de penitentia (C.33 q.3) in Gratian's Decretum, which became the textbook for canon law and served as the basis of the church's developing jurisprudence, in theory and in practice. This study examines the treatise in its entirety, providing a commentary on the content of the treatise and an examination of its relationship to the early twelfth-century schools.