The Chantilly 564 Codex is recognised as one of the most important sources of the late XIVth century. It contains 99 songs and 13 motets in French or Latin, many of which are unique and some written by composers whose names are not recorded anywhere else. Apart from the unusually rich repertoire, this manuscript provides many examples of the highly elaborate musical notation known as the Ars subtilior (varied colours, unusual presentation, first use of rhythmic structure), illustrating the way in which late XIVth century composers experimented with the written representation of musical rhythms. This manuscript is of exceptional interest. It probably originated from one of the regions now in the south of France or Spain and had an unusual history, passing over the centuries into the hands of some of the most famous collectors in the Italian peninsula. It resurfaced in modern times in Florence, after some four hundred years of oblivion, where it came to rest in the prestigious collection of Prince Henri d"Orleans, duke of Aumale (1822-1897). It now belongs to the Institut de France and is kept in the library of the Conde museum at the chateau de Chantilly. The photographs were taken by the Digital Image Archive of Medieval Music (University of Oxford, Royal Holloway, University of London), a particularly difficult task given the state of conservation of the work. The critical study introducing the manuscript is by Yolanda Plumley (University of Exeter) and Anne Stone (Queens College and the Graduate Center, City University of New York), a British and a Canadian musicologist specialising in XIVth and XVth music as well as in the musical notation of the Ars subtilior. Facsimile with English commentary.