Alongside his seminal work on lexicography, Hugh of Pisa dedicated much of his intellectual life to teaching at the University of Bologna, interpreting and commenting on Gratian's Decretum (or Concordantia discordantium canonum), the main source of medieval canon law. The Summa Decretorum, which bears the fruit of such activities, is one of the most authoritative expositions of canon law of the twelfth century. Its first 20 Distinctiones have now been published as the first volume of a critical edition of the Summa. This is a project of impressive dimensions, as it is expected to number 16 volumes as a whole. The idea of a critical edition of Huguccio's writings in canon law sprung from a group of scholars (namely Stickler, Catalano, Hizing, Leonardi, Prosdocimi, Schramml and Zanetti) who revolved around the Institute of Research and Study in Medieval Canon Law in Washington, DC. Their work began in the mid-1950s and was more recently handed over to Přerovský, the editor of this first volume. This book undoubtedly represents an invaluable resource for canon lawyers, legal historians and ecclesiastical historians alike, as, of course, does the project as a whole.