This thesis is an investigation of the manuscript of John Thomas, dated 1752 (NL W J. Lloyd Williams MS 39). The manuscript contains 526 musical items, as well as several lists of tune titles and two Welsh-language treatises on angling and musical theory. Little is known of the author other than that he came from North-East Wales. He appears to have been a man of educated interests who earned at least part of his living from fiddle playing. Around two-thirds of the tunes are written down from oral tradition, some of which are notated in an idiosyncratic form of violin notation probably invented by himself; the remainder are copied from various sources consisting mainly of country dance collections printed between 1718 and 1756. Most are tunes which were popular for dancing during the latter half of the seventeenth and first half of the eighteenth centuries. Others were used for Welsh-ianguage carolau and penillion singing. In addition, there are a few abstract instrumental pieces by well-known composers of the day. It is an eclectic collection -which reflects the widely-travelled nature of popular music at the time, including tunes from Wales, England, Scotland and beyond and music dating from previous centuries as well as more recent compositions. The manuscript provides an insight into the musical repertoire, performance practices and working life of a North Wales fiddler during the height of the popularity of the fiddle in Wales. It shows ways in which the fiddle was used to accompany dance and song and records versions of tunes shaped by fiddle players and Welsh singers. Seen in a wider musical and cultural context, it illuminates the state of the Welsh musical tradition at a fascinating point in its history.