  The primordial abundances of deuterium, helium-3, helium-4, and lithium-7
probe the baryon density of the Universe only a few minutes after the Big Bang.
Of these relics from the early Universe, deuterium is the baryometer of choice.
After reviewing the current observational status of the relic abundances (a
moving target!), the baryon density determined by big bang nucleosynthesis
(BBN) is derived. The temperature fluctuation spectrum of the cosmic background
radiation (CBR), established several hundred thousand years later, probes the
baryon density at a completely different epoch in the evolution of the
Universe. The excellent agreement between the BBN- and CBR-determined baryon
densities provides impressive confirmation of the standard model of cosmology,
permitting the study of extensions of the standard model. In combination with
the BBN- and/or CBR-determined baryon density, the relic abundance of 4He
provides an excellent chronometer, constraining those extensions of the
standard model which lead to a nonstandard early-Universe expansion rate.
