The papers in this volume are based on lectures given during the meeting of the Seminaire Sud Rhodanien de Geometrie which we organized at MSRI from May 22 to June 2, 1989, as part of a year-long program on Symplectic Geometry and Mechanics. The Seminaire Sud Rhodanien de Geometrie (SSRG) was established in 1982 by geometers and mathematical physicists at the Universities of Avignon, Lyon, Marseille, and Montpellier, with the aim of developing and coordinating research in symplectic geometry and its applications to analysis and mathematical physics. It has been designated by the Centre N ationale de la Recherche Scientifique as a "Groupement de Recherche" (G.D.R. 144), centered at the Universite Claude Bernard (Lyon I). From the beginning, the SSRG has involved the cooperation of colleagues from other universities inside and outside France; in addition to the editors of this volume, its Scientific Committee consists of D. Bennequin, P. Libermann, A. Lichnerowicz, C.-M. MarIe, J.-M. Morvan, P. Molino, and J.-M. Souriau. In particular, there have always been strong connections with the University of California at Berkeley, making this other "UCB" into a virtual fifth pole of the SSRG. Through its international meetings, of which the first five were held at Lyon, Montpellier, and Marseille, the SSRG has become an important cen ter of exchange for the latest developments in symplectic geometry and its applications. It seemed natural, therefore, to have this sixth meeting at MSRI in Berkeley in conjunction with the "symplectic year" 1988-89."