Among medieval Christian societies, Byzantium is unique in preserving an ecclesiastical ritual of adelphopoiesis that pronounces two men as brothers. It has its origin as a spiritual blessing in the monastic world of late antiquity, and it becomes a popular social networking strategy among lay people from the ninth century onwards, even finding application in recent times. Located at the intersection of religious and social history, brother-making exemplifies how social practice can become ritualized and subsequently subjected to attempts of ecclesiastical and legal control. Wide-ranging in its use of sources, from a complete census of the manuscripts containing the ritual of adelphopoiesis to the literature and archaeology of early monasticism, and from the works of hagiographers, historiographers, and legal experts in Byzantium to comparative material in the Latin West and the Slavic world, this book is the first exhaustive treatment of the phenomenon.