  We evaluate the role of radiation pressure in the dynamics of HII regions. We
first determine under what conditions radiation pressure is significant in
comparison to gas pressure and show that, while radiation pressure is generally
unimportant for HII regions driven by a handful of massive stars, it is
dominant for the larger HII regions produced by the massive star clusters found
near the Galactic center and in starburst environments. We then provide a
solution for the problem of how HII regions expand when radiation pressure
influences their behavior. Finally, we compare radiation-dominated HII regions
to other sources of stellar feedback, and argue that HII regions are probably
the primary mechanism for regulating the formation of massive star clusters.
