In his last monumental organ piece, “Livre du Saint Sacrement,” the French composer Olivier Messiaen (1908–92) approaches the mystery of the eucharistic presence, using all elements of his extraordinary compositional language: Gregorian chants, birdsongs, Greek and Hindu rhythms, serialism, and sound-colors. In this book, the "Livre du Saint Sacrement," which premiered in 1986, is fully analyzed and theologically interpreted for the first time. The influence of the eucharistic hymns of Thomas Aquinas, among other sources, on the work’s conception and the composer’s theology is explored, and the author points out the ways in which Messiaen’s musico-theological dedication to the Eucharist can inspire actual theology. The original dissertation that was the basis for this book received the Award of Excellence from the Austrian Ministry for Science, Research, and Economy, as well as the Roland-Atefie-Prize from the Austrian Academy of Sciences. For the first time, the book is now available in English translation.