This monograph addresses the history of canon law in Western Europe between ca. 1000 and ca. 1150, specifically the collections compiled and the councils held in that time. The main part consists of an analysis of all major collections, taking into account their formal and material sources, the social and political context of their origin, the manuscript transmission, and their reception more generally. As most collections are not available in reliable editions, a considerable part of the discussion involves the analysis of medieval manuscripts. Specialized research is available for many but not all these works, but tends to be scattered across miscellaneous publications in English, German, French, Italian, and Spanish; one purpose of the book is thus to provide relatively uniform, up-to-date accounts of all major collections of the period. At the same time, the book argues that the collections are much more directly influenced by the social milieux from which they emerged, and that more groups were involved in the development of high medieval canon law than it has previously been thought. In particular, the book seeks to replace the still widely held belief that the development of canon law in the century before Gratian’s Decretum (ca. 1140) was largely driven by the Reform papacy. Instead, it is crucial to take into account the contribution of bishops, monks, and other groups with often conflicting interests. Put briefly, local needs and conflicts played a considerably more important role than central (papal) ‘reform’, on which older scholarship has largely focused.