  Until recently, simulations that modeled entire galaxies were restricted to
an isothermal or fixed 2- or 3-phase interstellar medium (ISM). This obscured
the full role of the ISM in shaping the observed galactic-scale star formation
relations. In particular the Kennicutt relation suggests that star formation
rates depend in a simple way on global galactic quantities, such as mean gas
mass surface density and dynamical time. Contrary to this, observations of
nearby star-forming regions, including images from the Spitzer telescope, show
that all the way down to ``local-scales'' ($\sim$ parsecs) star formation is a
highly clustered process with the gas existing at a wide range of densities and
multiple phases, and seemingly decoupled from the larger-scale Galaxy. Many
different physical processes appear to be influencing the star formation rate,
including heating and cooling of the gas, ``turbulent'' energy injection from
outflows, winds and supernova blastwaves, ionization, and magnetic field
support. Here we present results from global simulations of disk galaxy star
formation that include a fully multiphase ISM and compare to simpler isothermal
models. We also investigate the effect of background heating. We discuss the
process of giant molecular cloud formation as one of the possible links between
the galactic and local scales of star formation.
