HI 21 cm probes of reionization, and beyond

I review the potential for observing cosmic reionization using the HI 21 cm line of neutral hydrogen. Studies include observations of the evolution of large scale structure of the IGM (density, excitation temperature, and neutral fraction), through HI 21 cm emission, as well as observations of small to intermediate scale structure through absorption toward the first discrete radio sources. I summarize predictions for the HI signals, then consider capabilities of facilities being built, or planned, to detect these signals. I also discuss the significant observational challenges. (c) 2005 Elsevier B.V. All rights reserved.