  In popular cold dark matter cosmological scenarios, stars may have first
appeared in significant numbers around a redshift of 10 or so, as the gas
within protogalactic halos with virial temperatures in excess of 20,000 K
(corresponding to masses comparable to those of present--day dwarf ellipticals)
cooled rapidly due to atomic processes and fragmented. It is this `second
generation' of subgalactic stellar systems, aided perhaps by an early
population of accreting black holes in their nuclei, which may have generated
the ultraviolet radiation and mechanical energy that ended the cosmic ``dark
ages'' and reheated and reionized most of the hydrogen in the cosmos by a
redshift of 6. The detailed history of the universe during and soon after these
crucial formative stages depends on the power--spectrum of density fluctuations
on small scales and on a complex network of poorly understood `feedback'
mechanisms, and is one of the missing link in galaxy formation and evolution
studies. The astrophysics of the epoch of first light is recorded in the
thermal state, ionization degree, and chemical composition of the intergalactic
medium, the main repository of baryons at high redshifts.
