In medieval Britain the seal was, for some three hundred years, the way most documents were authenticated. Seals and seal impressions are useful as evidence in problems of social and economic history, the history of art, law, and diplomacy, but to date they have not been thoroughly discussed. In this groundbreaking work the authors examine every kind of seal used for authenticating documents - their development, design, and use - with reference to contemporary records. An index of mottoes and inscriptions provides invaluable and practical help in identifying seals in other collections. The volume is abundant with illustrations, mainly depicting seals from the extensive collections of The British Library and the Public Record Office, London. Designed to fill a long-standing gap in the literature, this lucid introduction to British medieval seals is aimed at graduate students, archivists, historians, and anyone interested in medieval documents and artefacts.