The similarities in the way that the ubiquitous cult of saints was expressed in Wales, Cornwall and Brittany form a major body of evidence for contact among these regions. Overtly ecclesiastical place-names, containing such elements as *lann, are unusually common in all three regions. The implications of these for similarities and differences in church-organisation are discussed. The significance of the appearance of the same personal names compounded in ecclesiastical place-names in different British-speaking regions is explored. These seem likely in most cases to reflect movements of the devotees of particular saints between Wales, Cornwall and Brittany from the sixth to the ninth centuries and particularly the reliance of Bretons on the educational and spiritual resources of important Welsh churches. Contact between Brittany and Cornwall was more sustained and more intimate: a tenth-century list reveals a number of Breton saints established at permanent cult-sites in Cornwall by the tenth century and more were to follow by the central Middle Ages; Cornish and Breton authors also drew on each other’s work in composing saints’ Lives throughout the medieval period. By contrast the relative rarity of Irish saints’ cults in Brittany and the absence of Breton saints from Ireland implies a more distant relationship.