Why do people believe that they ought to obey particular legal rulings? 1 What qualities make a law binding? What is the source of legal authority? These questions were raised with fresh urgency in the eleventh century. This article examines the evolving nature of ‘authority’ in Church law by comparing the most important canonical collection of the eleventh century, the Decretum of Burchard, bishop of Worms, 2 to the Ivonian Decretum, which recent work has argued to be Ivo’s own collection. 3