Between the cosmic recombination at redshift z~1000 and the reionization at z~6-20, most of the baryons in the universe were in the form of neutral atomic gas. Several experiments are being built or proposed to directly observe this gas via the 21 cm hyperfine line of atomic hydrogen. This 21 cm signal contains a vast amount of information about the matter power spectrum, the sources of radiation that reionized the universe, and (via the lensing effect) the growth of large-scale structure at low redshifts. I will discuss the physics that determines what the 21 cm sky looks like, with an emphasis on how one can extract astrophysical and cosmological information. I will briefly discuss the observational hurdles that will have to be overcome in order for the promise of 21 cm radiation to be fully realized.