Charles Jennens (1700-1773), the librettist of 'Saul', 'Messiah', 'Belshazzar', the final part of 'L’Allegro, il Penseroso ed il Moderato', and probably 'Israel in Egypt', amassed a huge library of music that forms the bulk of what is now known as the Aylesford collection. Jennens’s collection of Handel’s music was unique among those of his contemporaries, not only because it includes part-books, but also because it is unusually comprehensive. The dissertation focuses on his copies of the sacred oratorios beginning with 'Saul' (1739) because most of the collection was copied in the 1740s, the sacred oratorios were the works that Jennens was most interested in, and 'Saul' was his first collaboration with Handel. As many of these manuscripts have not been the focus of modern scholarly attention, I first establish how, when, and by whom each manuscript was copied, in order to achieve a greater understanding of how and when Jennens assembled his collection, and what his reasons were for doing so. This close study of the manuscripts reveals that Jennens made extensive alterations to the verbal text, the structure, and the music of several oratorios in his collection. His amendments to 'Saul' and 'Belshazzar' shed light on his collaboration with Handel; and his amendments to 'Samson' and 'Joseph and his Brethren' provide insights into his attitude to Handel in the mid-1740s, his approach to word-setting, his views on the adaptation of Scripture for oratorio, and his beliefs and commitments. Jennens was a highly educated man whose activities were informed by two deeply held, conflicting allegiances: to the Anglican Church and to the deposed Stuarts. An examination of how he harnessed Handel’s music to deliver his religious and political messages leads to a richer and more profound understanding of the works, of the relationship between Jennens, Handel, and Handel’s music, and of their place in the religious and political context of the mid-eighteenth century.