Welsh poet R. S. Thomas (1913-2000), who published for over six decades, evolved from a sub-Georgian imitator in the 1930s to one of Britain's most stylistically-innovative poets in the latter part of the century. This thesis traces Thomas's developmental journey by means of close reading, analyzing his various prosodies, including metrical, accentual, linear, and visual. R. S. Thomas's prosodies were largely an extension of his emotional interiors, but there is also in his work an integral relationship between style and place; the environment in which he wrote poems was consistently reflected in the rhythm, language, and structure of the poetry itself. In order to shed light on the workings of Thomas's prosodies, this thesis makes use of poetic theory and examines Thomas's responses to key influences: the Bible; writings in theology and popular science; and the literary works of Irish, American, Welsh, and English writers.