  Among the light elements created in the Big Bang, deuterium is one of the
most difficult to detect but is also the one whose abundance depends most
sensitively on the density of baryons. Thus, although we still have only a few
positive identifications of D at high redshifts--when the D/H ratio was close
to its primordial value--they give us the most reliable determination of the
baryon density, in excellent agreement with measures obtained from entirely
different probes, such as the anisotropy of the cosmic microwave background
temperature and the average absorption of the UV light of quasars by the
intergalactic medium. In this review, I relate observations of D/H in distant
gas clouds to the large body of data on the local abundance of D obtained in
the last few years with the FUSE satellite. I also discuss some of the
outstanding problems in light element abundances and consider future prospects
for advances in this area.
