  Star formation is slow, in the sense that the gas consumption time is much
longer than the dynamical time. It is also inefficient; essentially all star
formation in local galaxies takes place in giant molecular clouds (GMCs), but
the fraction of a GMC converted to stars is very small, ~5%. In the most
luminous starbursts, the GMC lifetime is shorter than the main sequence
lifetime of even the most massive stars, so that supernovae can play no role in
GMC disruption. We investigate the disruption of GMCs across a wide range of
galaxies, from normal spirals to the densest starbursts; we take into account
the effects of HII gas pressure, shocked stellar winds, protostellar jets, and
radiation pressure produced by the absorption and scattering of starlight on
dust grains. In the Milky Way, we find that a combination of three mechanisms,
jets, HII gas pressure, and radiation pressure, disrupts the clouds. In more
rapidly star forming galaxies such as ``clump'' galaxies at high-redshift,
ultra-luminous infrared galaxies (ULIRGs) and submillimeter galaxies, radiation
pressure dominates natal cloud distribution. We predict the presence of 10-20
clusters with masses ~10^7 Msun in local ULIRGs such as Arp 220 and a similar
number of clusters with M_* ~ 10^8 Msun in high redshift clump galaxies;
submillimeter galaxies will have even more massive clusters. We find that the
mass fraction of a GMC that ends up in stars is an increasing function of the
gas surface density of a galaxy, reaching ~35% in the most luminous starbursts.
Furthermore, the disruption of bubbles by radiation pressure stirs the
interstellar medium to velocities of ~10 km/s in normal galaxies and to ~100
km/s in ULIRGs like Arp 220, consistent with observations. Thus, radiation
pressure may play a dominant role in the ISM of star-forming galaxies.
