  We argue that massive stars are the dominant sources of energy for the
turbulent motions within giant molecular clouds, and that the primary agent of
feedback is the expansion of H II regions within the cloud volume. This
conclusion is suggested by the low efficiency of star formation and
corroborated by dynamical models of H II regions. We evaluate the turbulent
energy input rate in clouds more massive than one third of a million solar
masses, for which gravity does not significantly affect the expansion of H II
regions. Such clouds achieve a balance between the decay of turbulent energy
and its regeneration in H II regions; summed over clouds, the implied ionizing
luminosity and star formation rate are roughly consistent with the Galactic
total. H II regions also photoevaporate their clouds: we derive cloud
destruction times somewhat shorter than those estimated by Williams and McKee.
The upper mass limit for molecular clouds in the Milky Way may derive from the
fact that larger clouds would destroy themselves in less than one crossing
time. The conditions within starburst galaxies do not permit giant molecular
clouds to be supported or destroyed by H II regions. This should lead to rapid
cloud collapse and the efficient formation of massive star clusters, explaining
some aspects of the starburst phenomenon.
