Three of Olivier Messiaen's later works, La Transfiguration de Notre-Seigneur Jésus-Christ, Méditations sur le mystère de la Sainte Trinité, and Saint François d'Assise, are linked by the fact that the composer refers to and quotes from Thomas Aquinas. The composer's reception of Thomistic texts is one of the principles guiding the interpretations in this study. On the one hand, Messiaen had been pondering Thomas's thoughts on the role of music in the life of a Christian and on music's possible spiritual content all through his professional life; on the other hand, the oratorio, the organ meditations, and the opera are the only works in which Messiaen quotes extensive Thomistic sentences addressing purely theological subject matter. The first aspect, Messiaen's appropriation of or felicitous congruence with the medieval theologian's views on music underlies all analyses as a kind of background fabric. The second aspect, Messiaen's quotations from the Summa theologica and their musical translation, determines segments of a larger discussion that, in the book's three main chapters, attempts to do justice to the compositions as a whole. While Thomas' theological aesthetics appears as a thread woven through a texture in a way that brings it only periodically to the foreground, the statements from Thomas's writings provide essential foundations determining the works' content and its musical rendering. This book is part of Siglind Bruhn's Messiaen Trilogy.