This essay offers a reconsideration of the idea of ‘The Great Century’ of Welsh literature (1435–1535) and related assumptions of periodization for understanding the development of lay piety and literature in fifteenth- and sixteenth-century Wales. It focuses on the origins of these ideas in (and their debt to) modern Welsh nationalist and Protestant and Catholic confessional thought, and their significance for the interpretation of Welsh literature and history. In addition, it questions their accuracy and usefulness in the light of contemporary patterns of manuscript production, patronage and devotional content of Welsh books of poetry and prose produced by the laity during and after this ‘golden age’ of literature. Despite the existence of over a hundred printed works in Welsh by 1660, the vernacular manuscript tradition remained robust; indeed, ‘native culture for the most part continued to be transmitted as it had been transmitted for centuries, orally or in manuscript’ until the eighteenth century. Bardic poetry’s value as a fundamental source for the history of medieval Ireland and Wales has been rightly acknowledged. However, more generally, Welsh manuscripts of both poetry and prose must be seen as a crucial historical source. They tell us much about contemporary views, interests and priorities, and offer a significant window onto the devotional world of medieval and early modern Welsh men and women. Drawing on recent work on Welsh literature, this paper explores the production and patronage of such books and the dynamics of cultural and religious change. Utilizing National Library of Wales Llanstephan MS 117D as a case study, it also examines their significance and implications for broader trends in lay piety and the nature of religious change in Wales.