These two volumes gather invited and contributed papers presented at the 21st International Symposium on Rarefied Gas Dynamics held in Marseille in 1998. These biennal symposiums constitute one of the main forums for presentation of recent advances in scientific and technical fields involving physical phenomena and processes in rarefied regimes. Thus, these volumes include numerous and various topics belonging to six general themes : Kinetic Theory and Transport, GasSurfaceInteraction and Phase Transition, Rarefied and Transitional Regimes, Monte-Carlo and Numerical Simulations, Nonequilibrium Flows and Plasmas, Aerospace Applications and Astrophysics. These books will interest scientists working in the analysis, modelisation and calculation of rarefied gas flows and corresponding processes in nonequilibrium conditions, as well as physicists and aerospace engineers trying to simulate and to compute complex reactive flows.