Bishops were of fundamental importance in the functioning of the medieval church, for they provided oversight of the work of the clergy and of education, of the parish system and of religious institutions of all kinds. This volume gathers together the evidence for the work of the bishops of the diocese of Ely, which comprised much of modern Cambridgeshire. It covers the early thirteenth century, and builds on an earlier volume in this series concerned with the twelfth century. The heart of this book is the edition of over 270 documents which show the work of the bishops and of the diocese. These are taken from very many archives and have never hitherto been accessible in one place; many have never been printed before or used by historians. They are accompanied by detailed commentary and references to supplementary documents elsewhere. The book also contains a substantial introduction, comprising a series of essays meant to support understanding of the documents themselves; this includes full biographies of the bishops, analysis of their administrative and domestic servants, and evaluation of the forms and types of documents in the collection. Alongside these is a long essay which examines the role of the bishops of Ely and their servants in the establishment of the University of Cambridge, which concludes that their roles have been much exaggerated. The edition and introduction are accompanied by detailed and extensive indexes to aid the user.