  There are few data available with which to constrain the thermal history of
the intergalactic medium (IGM) following global recombination. Thus far, most
constraints flow from analyses of the Cosmic Microwave Background and optical
spectroscopy along a few lines of sight. However, direct study of the IGM in
emission or absorption against the CMB via the 1S hyperfine transition of
Hydrogen would enable broad characterization thermal history and source
populations. New generations of radio arrays are in development to measure this
line signature. Bright foreground emission and the complexity of instrument
calibration models are significant hurdles. How to optimize these is uncertain,
resulting in a diversity in approaches. We discuss recent limits on line
brightness, array efforts including the new Large Aperture Experiment to Detect
the Dark Ages (LEDA), and the next generation Hydrogen Reionization Array
(HERA) concept.
