The Dragmaticon, William of Conches’ major systematic treatise on natural philosophy, was probably written in Normandy between 1144 and 1148. It was copied and widely circulated in no less than 80 manuscripts for more than 400 years. It was even translated into Catalan in the fifteenth century. The editio princeps, published in 1567 in Strasburg by Josiah Rihel and reprinted in 1967, was prepared by the Italian physician and humanist Guglielmo Gratarolo, who used just one manuscript he bought in Padua, then collated against another owned by Rihel, and often “emended” (sometimes quite arbitrarily). The present edition, by another Italian, a philologist and classicist turned medievalist, provides for the first time a critical text, based on 40 selected manuscripts. A detailed introduction informs on William of Conches’ works and career, as well as title, dating, aims, and the manuscript transmission of the Dragmaticon. Two registers of apparatus give a full documentation of both the sources and/or parallels and the meaningful variant readings of the 40 manuscripts used. The improvements on Gratarolo's edition are quite substantial. The critical edition of the early Catalan translation, edited by a known Catalan philologist, accompanies the Latin edition, making this volume all the more valuable.