John Cassian, also known as John the Ascetic and John Cassian the Roman, was a Christian monk and theologian celebrated in both the Western and Eastern churches for his mystical writings. Cassian is noted for his role in bringing the ideas and practices of early Christian monasticism to the medieval West.

Christian monasticism is the devotional practice of Christians who live ascetic and typically cloistered lives that are dedicated to Christian worship. It began to develop early in the history of the Christian Church, modeled upon scriptural examples and ideals, including those in the Old Testament. It has come to be regulated by religious rules and, in modern times, the Canon law of the respective Christian denominations that have forms of monastic living. Those living the monastic life are known by the generic terms monks (men) and nuns (women). The word monk originated from the Greek μοναχός, itself from μόνος meaning 'alone'.