In this section we give a very brief review of the main criticisms
toward hydrodynamics. Simulations and experiments employing minimal
models (L. P. Kadanoff [77,122]) and discussion of basic
principles (I. Goldhirsch [96,93]) have
addressed the many ``weak points'' along the rigorous derivation of
hydrodynamics. In terms of experiments, this means that the range of
validity of the continuum descriptions given in the previous section
is restricted to values of the restitution coefficient not too far
from 
, low volume fractions and intensities of the external driving
not too strong. In the rest of this work the use of hydrodynamics will
be reduced to minimum, while concepts taken from the theory of
stochastic processes, dynamical systems, phase ordering kinetics and
linear response theory will become the relevant tools of theoretical
interpretations for the results of computer simulations.