Star formation is slow in the sense that the gas consumption time is much longer than the dynamical time. It is also inefficient; star formation in local galaxies takes place in giant molecular clouds (GMCs), but the fraction of a GMC converted to stars is very small, epsilon{sub GMC} approx 5%. In 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 H II 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, a combination of three mechanisms-jets, H II 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 disruption. We predict the presence of approx10-20 clusters with masses approx10{sup 7} M{sub sun} in local ULIRGs such as Arp 220 and a similar number of clusters with M{sub *} approx 10{sup 8} M{sub sun} in high redshift clump galaxies; submillimeter galaxies will have even more massive clusters. We find that epsilon{sub GMC} = piGSIGMA{sub GMC} c/(2(L/M{sub *})) for GMCs that are optically thin to far-infrared radiation, where SIGMA{sub GMC} is the GMC gas surface density. The efficiency in optically thick systems continues to increase with SIGMA{sub GMC}, but more slowly, reaching approx35% in the most luminous starbursts. The disruption of bubbles by radiation pressure stirs the interstellar medium (ISM) to velocities of approx10 km s{sup -1} in normal galaxies and to approx100 km s{sup -1} in ULIRGs like Arp 220, consistent with observations. Thus, radiation pressure may play a dominant dynamical role in the ISM of star-forming galaxies.