Future quasar absorption line studies will combine insights
gained from detailed analyses of individual systems with
conclusions drawn from the large statistical samples assembled
over cosmic time.
Evolution of the ensemble of absorption profiles generated by
the universal collective of intervening structures is a result
of the combined effects of numerous processes.
These include growth of structure, star formation, morphological
evolution of galaxies, galaxy mergers, and changes in the
extragalactic background radiation.
Here, we summarize the best present statistical data and likely
interpretations for the different classes of absorbers.
The number of lines per unit redshift for various
populations of absorbers is represented by a power law
dN/dz 
(1 +
z)
. For a
universe with only
the cosmological evolution due to expansion,
= 1.0
for deceleration parameter q0 = 0 and
= 0.5 for
q0 = 0.5.