  The last parameter of big-bang nucleosynthesis, the baryon density, is being
pinned down by measurements of the deuterium abundance in high-redshift
hydrogen clouds. When it is determined, it will fix the primeval light-element
abundances. D, ^3He and ^7Li will become ``tracers'' for the study of Galactic
and stellar chemical evolution, and big-bang nucleosynthesis will become an
even sharper probe of particle physics, e.g., the bound to the number of light
neutrino species will be tightened significantly. Two key tests of the
consistency of the standard theory are on the horizon: an independent,
high-precision determination of the baryon density from anisotropy of the
cosmic background radiation and a precision determination of the primeval
$^4$He abundance.
