  The light element abundances probe the baryon density of the universe within
a few minutes of the Big Bang. Of these relics from the earliest universe,
deuterium is the baryometer of choice. By comparing its primordial abundance
inferred from observations with that predicted by Big Bang Nucleosynthesis
(BBN), the early universe baryon density is derived. This is then compared to
independent estimates of the baryon density several hundred thousand years
after the Big Bang and at present, more than 10 billion years later. The
excellent agreement among these values represents an impressive confirmation of
the standard model of cosmology.

Astrophysics