Music was central to the medieval church’s public worship: it was the essential medium of the Mass and the Divine Office. In this new critical edition, T. J. H. McCarthy presents the Latin text and the first English translation of Aribo’s musical treatise, De musica and Sententiae. Written between 1070 and 1078, it is concerned with the workings of the liturgical music that Aribo and his contemporaries called Gregorian chant, and builds off of and responds to several contemporary treatises by Abbot Bern of Reichenau and his pupil Herman, Abbot William of Hirsau, Frutolf of Michelsberg, and Theoger of Metz. In the first new edition of the treatise in over sixty years, McCarthy addresses not only new approaches to the study of music history but newly discovered manuscripts of the treatise, paying careful attention to the diagrams that are integral to the coherence of the treatise.