This paper discusses the varying levels of influence that Gregorian chant exerted on the music of Olivier Messiaen. The religious symbolism of Catholic theology dictated the programmatic elements of Messiaen's works, and the music of the Roman Catholic liturgy functioned as an important symbol within his compositional language. This paper offers both a comprehensive discussion of the relationship between Gregorian chant and the music of Messiaen, as well as an examination of important chant paraphrases within his compositions.