This thesis seeks to determine the relevance of the term 'Welsh composer' to the life and music of Grace Williams (1906-77). It first examines her relationship with the various institutions that have shaped the cultural life of Wales during the twentieth century, questioning particularly whether either party was able to exploit the resources of the other for their own and/or the wider Welsh people's benefit. The Welsh Region of the BBC is discussed in particular detail, but the impact of the policies and practices of Wales's Arts Council, National Eisteddfod, institutions of higher education, and performing artists upon Williams's output are also analysed. The second half of the thesis proceeds to examine those of Williams's works openly identified by their composer as having been influenced by her Welsh heritage: those that include native folk melodies (Hen Walia, 1930; Fantasia on Welsh Nursery Tunes, 1940); depict landscape and history (Four Illustrations for the Legend of Rhiannon, 1939; Symphonic Impressions, 1943); set Welsh texts (Gogonedawg Arglwydd, 1939; Four Mediaeval Welsh Poems, 1962); use poetic structures (Penillion, 1955; Ballads, 1968); or have no acknowledged programme beyond their Welsh-inflected title (Castell Caernarfon, 1969; Missa Cambrensis, 1971). Where otherwise unavailable, a compact history of the composition of each work is supplied (see also the prefaces to the accompanying scholarly editions), but this latter portion of the thesis prioritises musical concerns ahead of matters historical. To conclude, Williams's reception history amongst British and Welsh critics and audiences is examined for evidence that her nationality influenced not only how she composed, and for whom, but how she was widely perceived. The thesis is explicitly designed to be read alongside the present writer's scholarly editions of Four Illustrations for the Legend of Rhiannon, Gogonedawg Arglwydd, Four Mediaeval Welsh Poems, Ballads, Castell Caernarfon and Missa Cambrensis (submitted alongside it and fulfilling the requirements of the PhD degree). The text itself, therefore, includes very few musical examples, pointing instead to the relevant bar references. The Appendix to the thesis comprises a catalogue of Williams's extant works, updating and correcting those previously prepared by the National Library of Wales, Aberystwyth, and by Malcolm Boyd in his Grace Williams (Cardiff: University of Wales Press, 1980). Further material relating to Williams can be found in the present writer's 'Ambition Overshadowed: Grace Williams's symphonies evaluated' (MPhil dissertation, University of Wales, Bangor, 2007).