This book is a biography of St. Leontios, who lived in the 12th century, written by one of his disciples in the early years of the 13th century. Leontios was Abbot of the celebrated Monastery of St. John the Theologian in the Aegean island of Patmos and later became Greek-Orthodox Patriarch of Jerusalem.This study provides a critical edition of the text of the Life, an English translation and an extensive commentary on various subjects such as history, language, theology, monasticism and prosopography.The Life of Leontios is an important and interesting hagiographical work, not only because of the historical information it provides, but also because it is one of the few existing Byzantine historiographies of this period.