A complete specification of the microstructure of a realistic
porous medium is impractical.
The present chapter discusses possibilities for characterizing
porous media without specifying a porous geometry in all its detail.
As emphasized in the introduction this is the main theoretical
task.
There are two general approaches:
One approach constructs simplified geometric models for
each specific porous medium of interest.
The other approach attempts to find general characterizations
for large classes of porous media which are less specific but
more widely applicable.
This latter approach will be discussed first.
