  We briefly review the use of UV absorption lines in the spectra of
low-redshift QSOs for the study of the physical conditions, metallicity, and
baryonic content of the low-z IGM, with emphasis on the missing baryons
problem. Current results on the statistics and baryonic content of intervening,
low-z O VI and Lya absorption-line systems are presented with some comments on
overlap between these two classes of absorbers and consequent baryon
double-counting problems. From observations of a sample of 16 QSOs observed
with the E140M echelle mode of STIS, we find 44 intervening O VI absorbers and
14 associated O VI systems [i.e, z(abs) ~ z(QSO)]. The implied number of
intervening O VI absorbers per unit redshift is dN/dz(O VI) = 23+/-4 for rest
equivalent width > 30 mA. The intervening O VI systems contain at least 7% of
the baryons if their typical metallicity is 1/10 solar and the O VI ion
fraction is <0.2. This finding is consistent with predictions made by
cosmological simulations of large-scale structure growth. Recently, a
population of remarkably broad Lya lines have been recognized in low-z quasar
spectra. If these Lya lines are predominantly thermally broadened, then these H
I absorbers likely harbor an important fraction of the baryons. We present and
discuss some examples of the broad Lya absorbers. Finally, we briefly summarize
some findings on the relationships between O VI absorbers and nearby
galaxies/large-scale structures.
