This is the first English translation of a text that Cahill identifies as the first formal commentary on Mark's gospel. Probably written in Ireland in the first half of the seventh century, the commentary was for almost 1000 years attributed to St. Jerome and as such exercised incalculable influence on subsequent commentary, including Aquinas's Catena Aurea. This translation will make this unique text available to readers who do not know Latin and will serve as an invaluable introduction to late patristic and early medieval biblical exegesis.