The next decade will see the synthesis of the various techniques for the study of galaxy evolution, through their stars and through their gas. Higher resolution quasar spectra will be obtained in the ultraviolet (with the Space Telescope Imaging Spectrograph (STIS) and with the Cosmic Origins Spectrograph (COS) on the HST, and later, hopefully, with a larger UV space telescope). It will then be possible to conduct a systematic analysis of the relationships between the different ionization species that trace the different phases of gas in 0.4 < z < 1.5 galaxies. In this redshift regime, comparisons to the detailed morphological structure and orientations of the absorbing galaxies is possible from HST images.
Invaluable insights into the origin of quasar absorption lines have been gleaned from absorption studies of nearby galaxies, for which it is possible to directly observe the processes that are involved. Making more observations of this type will be possible by discoveries of bright quasars that fall behind nearby galaxies. The discoveries of quasars in large surveys will also include multiple lines of sight behind distant absorption line systems which can be used to produce 3-D maps of the structures.
The interstellar medium of the Milky Way shows structure on sub-pc scales, and absorption features can only be resolved with resolution < 1 km s-1. Such a resolution will soon be available on 8m-class telescopes. This is important for separating blends and for looking for metallicity, ionization, and abundance pattern gradients along the line of sight.
The key low ionization transitions of Mg II and Fe II are shifted into the near-IR region of the spectrum for z > 2.5. Very soon, near-IR quasar spectra will be obtained at relatively high resolution (~ 20 km s-1). Also, IR-imaging, narrow-band techniques, and multi-object spectroscopy in the near-IR should provide much more information about absorbing galaxies at higher redshifts. This will extend evolutionary studies back to an epoch at which formation processes may be contributing significantly to evolution.