  Galaxy disk formation must incorporate the multiphase nature of the
interstellar medium. The resulting two-phase structure is generated and
maintained by gravitational instability and supernova energy input, which yield
a source of turbulent viscosity that is able to effectively compete in the
protodisk phase with early angular momentum loss of the baryonic component via
dynamical friction in the dark halo. Provided that star formation occurs on the
viscous drag time-scale, this mechanism provides a means of accounting for disk
sizes and radial profiles.
  The star formation feedback is self-regulated by turbulent gas
pressure-limited percolation of the supernova remnant-heated hot phase, but can
run away in gas-rich protodisks to generate compact starbursts. A simple
analytic model is derived for a Schmidt-like global star formation law in terms
of the cold gas volume density.
