  The universe was reionized by redshift z ~ 6 by a small fraction of the
baryons in the universe, which released energy following their condensation out
of a cold, dark, and neutral IGM into the earliest galaxies. The theory of this
reionization is a critical missing link in the theory of galaxy formation. Its
numerous observable consequences include effects on the spectrum, anisotropy
and polarization of the cosmic microwave background and signatures of
high-redshift star and quasar formation. This energy release also created
feedback on galaxy formation which left its imprint on the mass spectrum and
internal characteristics of galaxies and on the gas between galaxies long after
reionization was complete. Recent work suggests that the photoevaporation of
dwarf galaxy minihalos may have consumed most of the photons required to
reionize the currently-favored Lambda-CDM universe. We will review recent
developments in our understanding of this process.
