The Epoch of Reionization
  The Universe's dark ages end with the formation of the first generation of
galaxies. These objects start emitting ultraviolet radiation that carves out
ionized regions around them. After a sufficient number of ionizing sources have
formed, the ionized fraction of the gas in the Universe rapidly increases until
hydrogen becomes fully ionized. This period, during which the cosmic gas went
from neutral to ionized, is known as the Universe's Epoch of Reionization . The
Epoch of Reionization is related to many fundamental questions in cosmology,
such as properties of the first galaxies, physics of (mini-)quasars, formation
of very metal-poor stars and a slew of other important research topics in
astrophysics. Hence uncovering it will have far reaching implications on the
study of structure formation in the early Universe. This chapter reviews the
current observational evidence for the occurrence of this epoch, its key
theoretical aspects and main characteristics, and finally the various
observational probes that promise to uncover it. A special emphasis is put on
the redshifted 21 cm probe, the various experiments that are currently being
either built or designed, and what we can learn from them about the Epoch of
Reionization.
