The treatise on musica plana and musica mensurabilis written by Lambertus/Aristoteles is our main witness to thirteenth-century musical thought in the decades between the treatises of Johannes de Garlandia and Franco of Cologne. Most treatises on music of this century - except for Franco�s treatise on musical notation - survive in only a single copy; Lambertus�s Ars musica, extant in five sources, is thus distinguished by a more substantial and long-lasting manuscript tradition. Unique in its ambitions, this treatise presents both the rudiments of the practice of liturgical chant and the principles of polyphonic notation in a dense and rigorous manner like few music treatises of its time - a conceptual framework characteristic of Parisian university culture in the thirteenth century. This new edition of Lambertus�s treatise is the first since Edmond de Coussemaker�s of 1864. Christian Meyer�s meticulous edition is displayed on facing pages with Karen Desmond�s English translation, and the treatise and translation are prefaced by a substantial introduction to the text and its author by Christian Meyer, translated by Barbara Haggh-Huglo.