  The last few years have seen a dramatic improvement in our understanding of
the origin of Lyman alpha absorption systems. Hydrodynamic numerical
simulations of cold dark matter dominated universes have shown that the many
properties of the Lyman alpha absorption systems can be explained by a
photoionized, space-filling, intergalactic medium. Lyman alpha lines offer
promising probes of the photoionizing background, the amplitude of the mass
fluctuations at high redshift and the evolution of the equation of state of the
intergalactic medium.
