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. Ce volume des articles de Michel Huglo termine la serie de quatre dans la collection Variorum. Il est centre sur la theorie musicale medievale. Le point de depart des recherches de Michel Huglo sur la theorie musicale du Moyen A'ge est forme par sa these sur les tonaires, editee en 1971: en consequence il etudia les manuscrits de theorie musicale concernant le plain-chant et, plus tard, les auteurs de l'Antiquite tardive et le Liber glossarum qui contient des definitions de nombreux termes musicaux. Dans ce volume, certains articles traitent de l'interpretation ou de la dissemination des textes, des instructions sur l'art du chant, et sur l'enseignement de la musique A l'Universite. Ils concernent les manuscrits du De musica d'Augustin, de Calcidius, Macrobe, Helisachar, Hucbald, Gerbert d'Aurillac, Abbon de Fleury, Jean d'Afflighem, Hieronymus de Moravia, et d'autres auteurs. Enfin, ce volume contient la bibliographie de