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...