  MHD turbulence plays a central role in the physics of star-forming molecular
clouds and the interstellar medium. I here show that MHD turbulence in
molecular clouds must be driven to account for the observed supersonic motions
in the clouds, as even strongly magnetized turbulence decays quickly. I then
show that driven MHD turbulence can globally support gravitationally unstable
regions, but local collapse inevitably occurs. Differences in the strength of
driving and the gas density may explain the very different rates of star
formation observed in different galaxies. The driving mechanism for the
turbulence is likely a combination of field supernovae in star-forming sections
of galactic disks, and magnetorotational instabilities in outer disks and low
surface brightness galaxies. Supernova-driven turbulence has a broad range of
pressures with a roughly log-normal distribution. High-pressure, cold regions
can be formed even in the absence of self-gravity.
