  Concerted effort is currently ongoing to open up the Epoch of Reionization
(EoR) ($z\sim$15-6) for studies with IR and radio telescopes. Whereas IR
detections have been made of sources (Lyman-$\alpha$ emitters, quasars and
drop-outs) in this redshift regime in relatively small fields of view, no
direct detection of neutral hydrogen, via the redshifted 21-cm line, has yet
been established. Such a direct detection is expected in the coming years, with
ongoing surveys, and could open up the entire universe from $z\sim$6-200 for
astrophysical and cosmological studies, opening not only the EoR, but also its
preceding Cosmic Dawn ($z\sim$30-15) and possibly even the later phases of the
Dark Ages ($z\sim$200-30). All currently ongoing experiments attempt
statistical detections of the 21-cm signal during the EoR, with limited
signal-to-noise. Direct imaging, except maybe on the largest (degree) scales at
lower redshifts, as well as higher redshifts will remain out of reach. The
Square Kilometre Array(SKA) will revolutionize the field, allowing direct
imaging of neutral hydrogen from scales of arc-minutes to degrees over most of
the redshift range $z\sim$6-28 with SKA1-LOW, and possibly even higher
redshifts with the SKA2-LOW. In this SKA will be unique, and in parallel
provide enormous potential of synergy with other upcoming facilities (e.g.
JWST). In this chapter we summarize the physics of 21-cm emission, the
different phases the universe is thought to go through, and the observables
that the SKA can probe, referring where needed to detailed chapters in this
volume (Abridged).
