  One of the milestones in the cosmic history is the formation of the first
luminous objects and Hydrogen reionization. The standard theory of cosmic
structure formation predicts that the first generation of stars were born about
a few hundred million years after the Big Bang. The dark Universe was then lit
up once again, and eventually filled with ultraviolet photons emitted from
stars, galaxies, and quasars. The exact epoch of the cosmic reionization and
the details of the process, even the dominant sources, are not known except the
fact that the universe was reionized early on. Signatures of reionization are
expected to be imprinted in the cosmic microwave background radiation,
especially in its large scale polarization. Future CMB experiments, together
with other probes such as 21 cm surveys, will provide rich information on the
process of reionization. We review recent studies on reionization. The
implications from available observations in a wide range of wavelengths are
discussed. Results from state-of-the-art computer simulations are presented.
Finally, we discuss prospects for exploring the first few hundred million years
of the cosmic history.
