  The basis for a hydrodynamic description of granular gases is discussed for a
low density gas of smooth, inelastic hard spheres. The more fundamental
mesoscopic description is taken to be the nonlinear Boltzmann kinetic equation.
Two arguments are presented in favor of a hydrodynamics for granular gases. The
first one is the concept of a "normal" solution and its explicit approximate
construction via the Chapman-Enskog method. The second is the demonstration of
hydrodynamic modes in the spectrum of the generator for the dynamics of small
spatial perturbations of the homogeneous reference state. In the first case, a
derivation of the nonlinear hydrodynamic equations is given to Navier-Stokes
order, with explicit expressions for the transport coefficients. The approach
is formal and the context of the derivation is left unspecified, although
internal mathematically consistency is demonstrated. The second method is more
restricted, leading only to linearized hydrodynamics, but with the potential to
define more sharply the context of hydrodynamics.
