  In cosmological models favored by current observations, the first
astrophysical objects formed in dark matter halos at redshifts starting at
z>20, and their properties were determined by primordial H_2 molecular
chemistry. These protogalaxies were very abundant, but substantially less
massive than typical galaxies in the local Universe. Extreme metal-poor stars,
and massive black holes in their nuclei reionized the bulk of the hydrogen in
the intergalactic medium. Reionization may have taken place over an extended
redshift interval, ending around z=7. Observational probes of the process of
reionization are afforded by studying the polarization of the cosmic microwave
background anisotropies, as well as by studying the spectra and abundance of
distant Lyman alpha-emitting galaxies. Here we review theoretical expectations
on how and when the first galaxies formed, and summarize future observational
prospects of probing hydrogen reionization.
