The 21cm Background: A Probe of Reionization and the Dark Ages

Observations of the redshifted 21cm line of neutral hydrogen have the potential to probe the processes of structure formation and reionization in a unique way, complementing other techniques in cosmology. The high redshift means that observations have to be done at frequencies of 200 MHz and below, a part of the spectrum plagued by radio frequency interference. I will review the status of the first-generation experiments that currently are under construction. Even modest collecting areas should be capable of detecting the power spectrum of fluctuations and the largest ``bubbles'' around quasars. Second generation experiments could in principle map structures on a wide range of spatial scales and, through sensitive power spectrum measurements, serve as a new probe of cosmological models and of dark matter.