This thesis provides a comprehensive source study of the eighteenth-century Chapel Royal partbooks (London, British Library R.M.27.a-d). The 56 manuscript volumes in this collection, which are now catalogued into four groups (or 'sets'), were used in the daily choral services at St James's Palace during the eighteenth and nineteenth centuries. The sources have a complex history since they have an 'organic' quality whereby the books continued to be copied into and altered whilst they were in regular use.The first part of the thesis (chapters two to six) examines the physical characteristics of the manuscripts by considering the books' construction, the traits of the copyists, and the way material was gradually added. Paper and scribal analysis, as well as general cataloguing work, are used to identify the contents and explore the layers of copying.The second part of the thesis (chapters seven and eight) looks at the function of the books and considers the collection within its eighteenth-century context. Documentary sources are considered alongside various elements of the books to establish how the partbooks were used in performance. The Chapel's method of partbook organisation is then compared with the organisation of similar collections at other choral foundations (including those with which the Chapel had strong connections).