  The development of primordial inhomogeneities into the non-linear regime and
the formation of the first bound objects mark the transition from a simple
cooling universe -- described by just a few parameters -- to a very messy hot
one -- the realm of radiative, hydrodynamic and star formation processes. In
popular cold dark matter cosmological scenarios, stars and quasars may have
first appeared in significant numbers around a redshift of 10 or so, as the gas
within subgalactic halos cooled rapidly due to atomic processes and fragmented.
This early generation of sources generated the ultraviolet radiation and
kinetic 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 thermal
history of the intergalactic medium -- the main repository of baryons at high
redshift -- 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.
