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.

Invited Paper, Plenary Session, to appear in Proceedings of the 20th Texas Symposium on Relativistic Astrophysics and Cosmology, eds. H. Martel and J. C. Wheeler, (AIP Conference Series), in press (2001); latex (aipproc.tex), 15 pages, 9 figures