  Primordial nucleosynthesis provides a probe of the Universe during its early
evolution. Given the progress exploring the constituents, structure, and recent
evolution of the Universe, it is timely to review the status of Big Bang
Nucleosynthesis (BBN) and to confront its predictions, and the constraints
which emerge from them, with those derived from independent observations of the
Universe at much later epochs in its evolution. Following an overview of the
key physics controlling element synthesis in the early Universe, the
predictions of the standard models of cosmology and particle physics are
presented, along with those from some non-standard models. The observational
data used to infer the primordial abundances are described, with an emphasis on
the distinction between precision and accuracy. These relic abundances are
compared with predictions, testing the internal consistency of BBN and enabling
a comparison of the BBN constraints with those derived from the WMAP Cosmic
Background Radiation data. Emerging from these comparisons is a successful
standard model along with constraints on (or hints of) physics beyond the
standard models of particle physics and of cosmology.
