The 21-cm Line as a Probe of Reionization
  One of the most exciting probes of the early phases of structure formation
and reionization is the spin-flip line of neutral hydrogen, with a rest
wavelength of 21 cm. This chapter introduces the physics of this transition and
the astrophysical parameters upon which it depends, including discussions of
the radiation fields that permeate the intergalactic medium that fix the
brightness of this transition. We describe the critical points in the evolution
of the 21-cm background and focus on the sky-averaged brightness and the power
spectrum as representative measurements. Finally, we include a discussion of
observations and the challenges they face in the near future.
