  We study the observability of the reionization epoch through the 21 cm
hyperfine transition of neutral hydrogen. We use a high-resolution cosmological
simulation (including hydrodynamics) together with a fast radiative transfer
algorithm to compute the evolution of 21 cm emission from the intergalactic
medium (IGM) in several different models of reionization. We show that the mean
brightness temperature of the IGM drops from dT_b~25 mK to dT_b~0.01 mK during
overlap (over a frequency interval of ~25 MHz), while the root mean square
brightness temperature fluctuations on small scales drop abruptly from ~10 mK
before overlap to ~0.1 mK at the end of overlap. We show that 21 cm
observations can efficiently discriminate models with a single early
reionization epoch from models with two distinct reionization episodes.
