  We present ongoing hydrodynamic and MHD simulations of molecular cloud
formation in spiral galaxies. The hydrodynamic results show the formation of
molecular gas clouds where spiral shocks compress atomic gas to high densities.
The spiral shocks also produce structure in the spiral arms, provided the gas
is cold (< 1000 K). When both hot and cold components of the ISM are modeled,
this structure is enhanced. Properties such as the clump mass spectra and
spatial distribution will be compared from clouds identified in these
simulations. In particular the multiphase simulations predict the presence of
much more interarm molecular gas than when a single phase is assumed. We also
discuss very recent results from galactic-scale MHD calculations. From
observational comparisons of the magnetic and thermal pressure, magnetic fields
are expected to be a major factor in explaining the dynamics of the ISM, from
kpc scales to those of star formation. We describe the difference in structure
of the spiral arms, and the evolution of the global magnetic field for a range
of field strengths.
