Surviving in three ninth-century manuscripts, the collection of canon law known as the Collectio 400 capitulorum is a remarkable and understudied witness to the scholarly vitality of the Carolingian period. Its 404 chapters offer ecclesiastical rules and moral guidelines taken from an unusually wide variety of authoritative sources. In addition to the customary canonical texts, such as the acts of the ecumenical councils and papal letters, the compiler of this collection drew his canons from the bible, Roman law, local Gallic synods, the Church Fathers, as well as Frankish and Insular penitential works.Although the Collectio 400 capitulorum is a so-called systematic collection, eminent scholars of canon law commented on its lack of structure. Even 'with the best will in the world', the collection's system eluded them. Despite its flaws, however, there is evidence that the collection gained some popularity in the ninth century, apparently providing the basis for the Poenitentiale Martenianum directly or indirectly influencing Hrabanus Maurus and Benedictus Levita. The ninth-century appreciation is understandable for, as one of the many products of the vigorous canonical activity of the eighth and ninth centuries, the Collectio 400 capitulorum impresses in its handling of the canonical material as well as the breadth of sources and the topics covered.This book constitutes the first in-depth study of this intriguing canonical collection, with a detailed description of the extant manuscript witnesses, its sources, and its influence. The critical edition offers scholars of the early Middle Ages in general and canon law in particular access to an instructive, if unpolished, product of Carolingian legal thought.