Olivier Messiaen's compositions for voice, though less widely known than his instrumental works, span some forty years and comprise a fifth of his total output. They have hitherto not been subject to much attention. A study of the elements comprising the vocal lines and accompanying instrumentation from the piano-voice song set, Trois melodies (1930), to the vast orchestral-choral La Transfiguration de Notre Seigneur Jesus-Chlist (1969) reveals, amongst other characteristic and evolving features, the emergence of a unique, simulated, plainchant style and its subsequent transformation into incantation with suitably modified accompaniment. While wide-ranging, chromatic and rhythmically free vocal lines are typical of many twentieth century compositions, Messiaen's use of such features is found to be novel, by virtue of the peculiar modal and temporal ambits within which he operated. Vocal delivery is progressively expanded from conventional bel canto production to humming, howling,and eventually speech and percussive sounds and reaches an apotheosis in the virtuoso effects of Cinq Rechants (1949). This recedes somewhat in La Transfiguration, which displays instead a wealth of hybrid plainsong-type writing. Choral works are interesting in that the emphasis shifts from standard part-writing to monody or accompanied unison singing, with an attendant absence of characterised solo parts. By constantly varying the colour of the single melodic line with different permutations of voice types, timbre assumes a new importance, particularly in La Transfiguration. The study of the texts, most of which were conceived by the composer simultaneously with the music, contributes much to the understanding of each work. Biblical symbolism in the early lyrics is progressively enriched by references to numerological, mythological and nature symbolism, mixed in an increasingly Surrealistic manner. The gradual incorporation of emotive phonemes in the texts, culminates in the invented language of Cinq Rechants. The thesis thus reveals an evolving yet persistently idiosyncratic vocal style, which establishes Messiaen as one of the most original composers of his time. It further demonstrates that his vocal works are an important component of his total oeuvre and also a significant contribution to twentieth century vocal literature.