  We review recent 3D high resolution numerical HD and MHD studies of a
multi-component and multi-phase ISM. The computational grid was chosen large
enough in the disk to represent typical ISM gas patterns, and in the halo to
include the full extent of the Galactic fountain. The evolution of the ISM is
driven by supernovae of all types and followed for 400 Myrs, long enough to get
rid of memory effects of the initial distributions. The results are
substantially different from previous classical analytical models and entail:
(i) a very inhomogeneous distribution of ISM gas, (ii) a high level of
supernova driven turbulence, (iii) an active Galactic fountain flow, reaching
out to about 5 kpc on either side of the disk, (iv) a low volume filling factor
for the hot gas of about 20%, (v) a very weak correlation between magnetic
field and gas density, n, for n < 100 cm^-3, and, (vi) about 50% of the gas
disk mass with n < 100 cm^-3, residing in classically thermally unstable
temperature regimes.
