The ordo officiorum of Montecassino is the earliest complete documentation of the liturgical practice of the Divine Office at the mother house of Benedictine monasticism. The document edited here is a comprehensive: directory of all the liturgical pieces performed at the abbey church of Montecassino throughout the year; to the central ordo is added a variety of other material, including precious information on the performance of" the liturgy and topographical references to altars, churches, and processions. Because of the importance of this document it was adapted many. times, for later use at the abbey and its daughter houses, and for use by. other monasteries, notably Sancta Sophia and St. Perer's monasteries in Benevento. Professor Kelly's edition compares the eight surviving witnesses of this text ( 11 th-13th centuries) and provides a substantial introduction to the sources and their significance.