This is the final volume in the set of four collections of Michel Huglo's articles to be published in the Variorum series, and focuses on medieval music theory. The point of departure for Huglo's research was his doctoral dissertation on tonaries, published in 1971: as a consequence, he studied the manuscripts of music theory concerning plainchant, and, later, those with writings on music by authors of Late Antiquity as well as the Liber glossarum, with its many definitions of musical terms. In this volume, certain articles consider the interpretation or dissemination of texts, instruction in the art of plainchant, and musical instruction at the university. Others concern the manuscripts of St Augustine's De musica and of the writings of Calcidius, Macrobius, Helisachar, Hucbald, Gerbert of Aurillac, Abbo of Fleury, John of Afflighem, and Hieronymus de Moravia, amongst others. The volume closes with a bibliography of Michel Huglo complementing that published in 1993 and a summary list of his reviews of books on music and liturgy.