  Spectroscopy of redshifted radio absorption of atomic and molecular species
provide excellent probes of the cold component of the gas in the early Universe
which can be used to address many important issues, such as measuring baryonic
content, probing large-scale structure and galaxy evolution, as well as
obtaining independent measurements of various combinations of fundamental
constants at large look-back times. However, such systems are currently very
rare with only 80 detected in HI 21-cm and five in OH and millimetre-band
species. Here we summarise the main selection criteria responsible for this and
how the next generation of radio telescopes are expected to circumvent these
through their wide instantaneous bandwidths and fields-of-view.
