The title of this article could of course encompass an entire meeting. I will focus my comments on reviewing of what we know about the most numerous absorption lines, the neutral hydrogen absorbers, and their evolution with redshift. This field of study has undergone a renaissance in last few years driven by observations with the Hubble Space Telescope of low redshift quasar absorption lines, observations of high redshift absorbers with the HIRES instrument on Keck, and cosmological modeling that allows us to make detailed comparisons of lines of sight through simulated universes. ©1999 American Institute of Physics.