When Olivier Messiaen completed the vast opera Saint Francois d'Assise in 1983, he was mentally and physically exhausted, believing that this monumental work would be his final compositional statement. Coaxed at first by his wife, Yvonne Loriod, he began to write a series of miniatures -a rarity in his output. Then, in the last years of his life, Messiaen composed a final large orchestral cycle, Eclairs sur l'Au-Dela ... This thesis examines the seven works which Messiaen completed after Saint Francois and argues that, following the crisis provoked by the opera, his music underwent a discernible change in style. In addition, the thesis uses the works in question to examine the characteristics of Messiaen's music, with a particular emphasis on an often overlooked aspect of his technique: harmony. In the process, many other questions are addressed, such as the ways in which Messiaen utilizes birdsong within a larger structure. The thesis is in three parts. Part I begins with a brief historical survey before discussing Saint Francois d'Assise as the work which defines everything that follows. Part II examines the series of miniatures whose significance belies their (relatively) modest proportions. Not only do they provide an indication of Messiaen's artistic self-confidence, but they also contain important links with his final masterpiece, Eclairs sur l'Au-Dela ... This sublime eleven movement work for large orchestra is the subject of Part III, and is the focus of the thesis. Each movement is analysed in turn, before the work is considered as an entity and its hidden structure and motivic cohesion is demonstrated.