The first principal part of this chapter explores sacred music in Wales from the Roman period to the English conquest. Christian observance in Wales may have been unbroken from the time of the Roman invasion onwards, and the four medieval cathedrals existed as sacred sites before Augustine’s mission of 578. Wales was part of the western Latin Church within the province of Canterbury. From the thirteenth century it was strongly influenced by the liturgical Use of Salisbury. However, only two substantial notated musical sources survive, both from the earlier fourteenth century: the Anian of Bangor Pontifical (with stronger provenance in East Anglia than Wales) and the Penpont antiphonal from the diocese of St Davids. This chapter considers their implications for our understanding of the repertoire and practice of sacred music in medieval Wales and also explores the role of the organ in the liturgy, drawing on the evidence of the pre-Reformation organ case at Old Radnor in Powys. The latter part of the chapter considers the effects of the Reformation on worship in Wales up to 1650. It examines compositions by John Lloyd, Philip ap Rhys and Elway Bevin, all of likely Welsh descent, and explains the significance of sources associated with Chirk Castle Chapel for understanding liturgical music in this period. After the Reformation, the English Book of Common Prayer was as alien as the Latin books it replaced in much of Wales. The Welsh translations of the Book of Common Prayer and the Bible were therefore crucial; the chapter concludes with an examination of Edmwnd Prys’ Welsh metrical psalter, Llyfr y Psalmau (1621).