At the level of atoms, using large-scale molecular-dynamics simulations, we have begun to elucidate the response of materials to shockwave loading and unloading. In dense monatomic, chemically unreactive fluids, the profile or structure of a shock wave is rather boring, being well described by viscous flow. In solids, on the other hand, the structure is far more complex, being dominated by plastic flow mechanisms that can even resemble phase transformations. We have just begun to explore the richness of the shock loading regime in crystalline solids, while much of the behavior upon unloading has heretofore remained in the province of speculation. We discuss some of the recent advances we have made at Los Alamos in simulations of shock waves and related phenomena, including plastic deformation, high-speed interfacial sliding, and fragmentation. As experimental observations become more and more refined, and molecular-dynamics simulations become larger, even approaching the mesoscale, fruitful overlap is achievable in the near future. ©2000 American Institute of Physics.