This article considers the theology of sin among Eastern ascetics of the early Byzantine period, exploring their theology’s focus, key points, and terminology. It highlights the main stages in the formation and development of monastic hamartiology by examining the contributions of three major ascetic schools: the schools of Egypt, Palestine, and Mount Sinai. Consideration of their key representatives (Evagrius Ponticus, John Cassian, Barsanuphius and John, Dorotheus of Gaza, and John Climacus) provides a panoramic view of the main streams in the development of monastic hamartiology and assesses its significance for the broader Christian doctrine of sin.