Sir John Prise (1501/2 - 1555), of Brecon, was an influential lawyer and administrator during the reigns of King Henry VIII, King Edward VI and Queen Mary I. In the 1530s he was brought under the aegis of Thomas Cromwell, to whose family he became connected by marriage, and was appointed visitor and commissioner for the dissolution of monasteries in England and Wales. The experience made him acutely aware of the wealth of manuscripts contained in these religious houses, and alone among the commissioners he set about saving material from their libraries. In 1540 he was appointed secretary of the Council in the Marches of Wales and made his home in Hereford, in the dissolved Benedictine Priory of St Guthlac: it remained his base for the last fifteen years of his life, a time in which he combined public duty with a deep commitment to literary and scholarly pursuits. In 1546 he was responsible for the printing of Yny lhyvyr hwnn, the earliest printed book in the Welsh language. His greatest work, however, is his Latin book, Historiae Britannicae Defensio. It is notable not only for its author's knowledge of British antiquity, founded on years of study of manuscript and other sources including - most importantly for Prise - material in Welsh, but also for the range of its learning, its lucid Latinity and the forensic quality of its arguments. The present work brings John Prise's Historiae Britannicae Defensio back into print for the first time since 1573. The facing English translation is the first published translation of the Defensio. The work is accompanied by an extensive introduction and elucidatory notes.