This volume, the fifth in the series, contains the proceedings from the conference Custom: The Development and Use of a Legal Concept in the Middle Ages held at the Law School at the University of Aarhus in May 2008. The volume covers topics from local case studies and studies of learned law to broader reflections on the development and use of the legal concept consuetudo and its connection with other sources of law, with the balance between local and regional power structures, and secular and ecclesiastical societies in medieval Europe. Combining the approaches of several historical disciplines - political, social, intellectual, and legal - international eminent scholars offer their views on central aspects of the function of legal customs and of the development of one of the most debated concepts in legal historiography of the last century. Students and scholars of European legal history and legal culture and of medieval history in general should find this collection of essays a useful contribution to the continuing discussion about the development of European law, legal principles and notions of justice.