  Star formation depends on the available gaseous "fuel" as well as galactic
environment, with higher specific star formation rates where gas is
predominantly molecular and where stellar (and dark matter) densities are
higher. The partition of gas into different thermal components must itself
depend on the star formation rate, since a steady state distribution requires a
balance between heating (largely from stellar UV for the atomic component) and
cooling. In this presentation, I discuss a simple thermal and dynamical
equilibrium model for the star formation rate in disk galaxies, where the basic
inputs are the total surface density of gas and the volume density of stars and
dark matter, averaged over ~kpc scales. Galactic environment is important
because the vertical gravity of the stars and dark matter compress gas toward
the midplane, helping to establish the pressure, and hence the cooling rate. In
equilibrium, the star formation rate must evolve until the gas heating rate is
high enough to balance this cooling rate and maintain the pressure imposed by
the local gravitational field. In addition to discussing the formulation of
this equilibrium model, I review the current status of numerical simulations of
multiphase disks, focusing on measurements of quantities that characterize the
mean properties of the diffuse ISM. Based on simulations, turbulence levels in
the diffuse ISM appear relatively insensitive to local disk conditions and
energetic driving rates, consistent with observations. It remains to be
determined, both from observations and simulations, how mass exchange processes
control the ratio of cold-to-warm gas in the atomic ISM.
