Metal abundances have been estimated for less than a dozen
absorption line systems. The main impediment is the scarcity of
systems which show enough lines from enough ions to allow
simultaneous estimates of both ionization and abundances.
Difficulties are also encountered with inadequate spectral
resolution, line blending, the existence of multiple
subcomponents, and slight differences in the velocity
distributions of the different ions in a given system. Published
abundance estimates range from near solar, to about 0.01 of
solar abundances. These estimates are sufficiently accurate to
show that abundances do differ, but they do not allow a
determination of the full range or distribution. Moreover they
are unlikely to be fully representative of all metal line
systems because estimates are usually made for only those
systems which show the largest number of metal lines, a bias
which will lead to above average abundances. Metal line systems
are sufficiently common that only a few percent can occur within
the Ho