In the late fourth century, in the absence of formal church councils, bishops from all over the Western Empire wrote to the Pope asking for advice on issues including celibacy, marriage law, penance and heresy, with papal responses to these questions often being incorportated into private collections of canon law. Most papal documents were therefore responses to questions from bishops, and not initiated from Rome. Bringing together these key texts, this volume of accessible translations and critical transcriptions of papal letters is arranged thematically to offer a new understanding of attitudes towards these fundamental issues within canon law. Papal Jurisprudence, c.400 reveals what bishops were asking, and why the replies mattered. It is offered as a companion to the forthcoming volume Papal Jurisprudence: Social Origins and Medieval Reception of Canon Law, 385–1234.