  Recent observations have set the first constraints on the epoch of
reionization (EoR), corresponding to the formation epoch of the first luminous
objects. Studies of Gunn-Peterson (GP) absorption, and related phenomena,
suggest a qualitative change in the state of the intergalactic medium (IGM) at
$z \sim 6$, indicating a rapid increase in the neutral fraction of the IGM,
from $x_{HI} < 10^{-4}$ at $z \le 5.5$, to $x_{HI} > 10^{-3}$, perhaps up to
0.1, at $z \ge 6$. Conversely, transmission spikes in the GP trough, and the
evolution of the \lya galaxy luminosity function indicate $x_{HI} < 0.5$ at
$z\sim 6.5$, while the large scale polarization of the cosmic microwave
background (CMB) implies a significant ionization fraction extending to higher
redshifts, $z \sim 11 \pm 3$. The results suggest that reionization is less an
event than a process, with the process beginning as early as $z \sim 14$, and
with the 'percolation', or 'overlap' phase ending at $z \sim 6$. The data are
consistent with low luminosity star forming galaxies as being the dominant
sources of reionizing photons. Low frequency radio telescopes currently under
construction should be able to make the first direct measurements of HI 21cm
emission from the neutral IGM during the EoR, and upcoming measurements of
secondary CMB temperature anisotropy will provide fine details of the dynamics
of the reionized IGM.
