Measurement of the spatial distribution of neutral hydrogen via the
redshifted 21 cm line promises to revolutionize our knowledge of the epoch of
reionization and the first galaxies, and may provide a powerful new tool for
observational cosmology from redshifts 1<z<4 . In this review we discuss recent
advances in our theoretical understanding of the epoch of reionization (EoR),
the application of 21 cm tomography to cosmology and measurements of the dark
energy equation of state after reionization, and the instrumentation and
observational techniques shared by 21 cm EoR and post reionization cosmology
machines. We place particular emphasis on the expected signal and observational
capabilities of first generation 21 cm fluctuation instruments.Comment: Invited review for Annual Review of Astronomy and Astrophysics (2010
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