  The absorption lines observed in quasar spectra have given us a detailed
picture of the intergalactic medium and the metal abundance and kinematics of
high redshift galaxies. In this review, we present an introduction to the
field, starting with the techniques used for interpreting absorption line
spectra. We then survey the observational and theoretical development of our
understanding of the Lyman-alpha forest, the metal absorbers, and the damped
Ly-alpha absorbers. We conclude with a discussion of some of the remaining
outstanding issues, and prospects for the future.
