  Lyman-alpha absorption is a prominent cosmological tool for probing both
galactic halos and the intergalactic medium at high redshift. We consider a
variety of sources that can be used as the Lyman-alpha emitters for this
purpose. Among these sources, we argue that quasars are the best probes of the
evolution of massive halos, while gamma-ray bursts represent the cleanest
sources for studying the reionization of the intergalactic medium.
