The historical, political, ecclesiastical, and religious relationships between medieval Cornwall, Brittany, Wales, Ireland and England are explored here through a study of the cult of St Petroc. Evidence for the cult in each area is thoroughly surveyed, but Cornwall and Brittany, the most important loci of the cult and most closely linked by language and culture, are the book's primary focus. The implications of the cult of a Celtic saint (generally an intensely local phenomenon) shared between Cornwall and Brittany are discussed, and attention is given to the highly politically-directed twelfth-century account of the furtive translation of the saint's relics to Brittany, which offers invaluable evidence for relations between Cornwall and Brittany, and also for Brittany's position in the Angevin empire of Henry II. Dr KAREN JANKULAK lectures in the Department of Welsh, University of Wales, Lampeter. She gained her Ph.D. from the Centre for Medieval Studies, Toronto, and was a post-doctoral fellow at the School of Celtic Studies, Dublin Institute for Advanced Studies.