  Many features of real granular fluids under rapid flow are exhibited as well
by a system of smooth hard spheres with inelastic collisions. For such a
system, it is tempting to apply standard methods of kinetic theory and
hydrodynamics to calculate properties of interest. The domain of validity for
such methods is a priori uncertain due to the inelasticity, but recent
systematic studies continue to support the utility of kinetic theory and
hydrodynamics as both qualitative and quantitative descriptions for many
physical states. The basis for kinetic theory and hydrodynamic descriptions is
discussed briefly for the special case of a low density gas.
