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.
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