The reader will feel the inspiration, freshness and enthusiasm of a new breakthrough in mathematical thought in the 4 seminal papers presented here for the first time ever in one volume. The 4 papers by Duistermaat, Guillemin and Hörmander have been a milestone in the development of this field. The new ideas presented in these papers - in a very lucid and accessible form - became the foundation on which more and more abstract theories were built in the relentless march towards even better and more detailed results. The detailed introductory comments by V. Guillemin are the final touch in a volume that combines the basic motivation and ideas of the early sources with the present state of F.I.O. thus bridging the gap between the past and present.