This article investigates the religious practice of suffering for others in the early Middle Ages. In proxy penance, one person completed a penitential work for another, who received the spiritual benefit. This practice was based on the idea that one person could stand in for another to bear his burden. Using penitential, conciliar, liturgical, and epistolary sources, I uncover two types of proxy penance. First, priests shared in the penance of those who confessed to them. Liturgical texts include Masses in which the priest completes the penance for someone who could not complete it himself. Penitential texts admonish the priest to “share in the foulness” with the sinner in order to bring about the remission of his sin. Second, there was both a promotion and a criticism of proxy fasting among the laity. This sic et non rhythm shows that early medieval penitential culture could not control the demand for proxy penance. Some attention is also paid to the practice of proxy penance in the eleventh-century monastic milieu of Peter Damian. This article broadens the scope of current scholarship on penance by focusing on its substitutionary ability. Also, this article explores the changing notions of and metaphors about sin in this period—from medical to economic—that fueled proxy activity.