  Fluid dynamics corresponds to the dynamics of a substance in the long
wavelength limit. Writing down all terms in a gradient (long wavelength)
expansion up to second order for a relativistic system at vanishing charge
density, one obtains the most general (causal) equations of motion for a fluid
in the presence of shear and bulk viscosity, as well as the structure of the
non-equilibrium entropy current. Requiring positivity of the divergence of the
non-equilibrium entropy current relates some of its coefficients to those
entering the equations of motion. I comment on possible applications of these
results for conformal and non-conformal fluids.
