  Massive stars produce copious quantities of ultraviolet radiation beyond the
Lyman limit, photoionizing the interstellar medium (ISM) and producing H II
regions. As strong sources of recombination- and forbidden-line emission,
infrared continuum, and thermal (free-free) radio continuum, H II regions serve
as readily-observable beacons of massive star formation in the Milky Way and
external galaxies. Along with supernovae, H II regions are dominant sources of
feedback in star-forming galaxies, injecting radiative and mechanical
luminosity into the ISM. H II regions may prove more important than supernovae
as triggers of star formation through localized compression of cold cloud
cores. In this review, I give a broad overview of the structure and
time-evolution of H II regions, emphasizing complications to the theoretical
picture revealed by multiwavelength observations. I discuss a recent
controversy surrounding the dominant feedback mechanism in 30 Doradus, the most
luminous H II region in the Local Group. I summarize the first results from the
Milky Way Project (MWP), which has produced a new catalog of several thousand
candidate Galactic H II regions by enlisting >35,000 "citizen scientists" to
search Spitzer Space Telescope survey images for bubble-shaped structures. The
MWP and similar large catalogs enable empirical studies of Galactic H II region
evolution across the full range of luminosities and statistical studies of
triggered star formation.
