  We discuss the prospects for ``tomography'' of the intergalactic medium (IGM)
at high redshifts using the 21 cm transition of neutral hydrogen. Existing
observational constraints on the epoch of reionization imply a complex
ionization history that may require multiple generations of sources. The 21 cm
transition provides a unique tool to probe this era in detail, because it does
not suffer from saturation effects, retains full redshift information, and
directly probes the IGM gas. Observations in the redshifted 21cm line will
allow one to study the history and morphology of reionization in detail.
Depending on the characteristics of the first sources, they may also allow us
to probe the era before reionization, when the first structures and luminous
sources were forming. The construction of high signal-to-noise ratio maps on
arcminute scales will require approximately one square kilometer of collecting
area.
