English summary: Olivier Messiaen was one of the great composers and organists of the twentieth century. The present volume presents his conversations with Olivier Glandaz, the man responsible for maintaining his organ, concerning the importance of the instrument and its repertoire. French description: Olivier Messiaen (1908-1992), compositeur et organiste, se confie a l'organier Olivier Glandaz. Pendant plus de vingt ans, celui-ci l'a accompagne pour l'entretien de son orgue a l'eglise de la Trinite. Ce document inedit revele l'importance capitale que representait l'orgue, l'instrument et son repertoire, pour le grand compositeur.