The papers in this volume are an outgrowth of the third "International Symposium on Hamiltonian Systems and Celestial Mechanics", HAMSYS-98, which was held in Patzcuaro, Michoacan, Mexico, December 7 to 11, 1998. The main topics of the talks were Arnold diffusion, central configurations, singularities in few body problems, billiards, area preserving maps, and geometrical mechanics.