Guillaume Durand, or William Durand (c. 1230 – November 1, 1296), also known as Durandus, Duranti or Durantis, from the Italian form of Durandi filius, as he sometimes signed himself, was a French canonist and liturgical writer, and Bishop of Mende.
He was born at Puimisson, near Béziers, of a noble family of Languedoc. He studied law at Bologna, with Bernardus of Parma, and by about 1264 was teaching Canon law with success at Modena. Pope Clement IV, another Frenchman, called him to the pontifical court as a chaplain and auditor of the palace, and in 1274 he accompanied Clement's successor, Pope Gregory X, to the Second Council of Lyons, the constitutions of which he helped draw up. As spiritual and temporal legate of the patrimony of St. Peter, he received in 1278, in the name of the pope, the homage of Bologna and of the other cities of Romagna. Pope Martin IV made him vicar spiritual in 1281, then governor of Romagna and of the March of Ancona (1283). In the midst of the struggles between Guelfs and Ghibellines, Durandus successfully defended the papal territories, both by diplomacy and by arms. Pope Honorius IV retained him in his offices, and although elected bishop of Mende in 1286, he remained in Italy until 1291. In 1295 he refused the archbishopric of Ravenna, offered him by Pope Boniface VIII, but accepted the task of pacifying his former provinces of Romagna and the March of Ancona. In 1296 he withdrew to Rome, where he died. His tomb is in the church of Santa Maria sopra Minerva.
Durandus' principal work is the Speculum iudiciale, which was compiled in 1271, and revised in 1286 and 1291. It is a general explanation of civil, criminal and canonical procedure, and also includes a survey of the subject of contracts. It is a remarkable encyclopedical synthesis of Roman and ecclesiastical law, distinguished by its clarity, its method, and especially its practical sense, and its repute was as great and lasting in the courts as in the schools.