  A critical review is given of the current status of cosmological
nucleosynthesis. In the framework of the standard model with 3 types of
relativistic neutrinos, the baryon-to-photon ratio, \eta, corresponding to the
inferred primordial abundances of helium-4 and lithium-7 is presently ~2 \sigma
below the value implied by the abundance of deuterium. The latter value is also
coincident with the independent determination of \eta from WMAP observations of
CMB anisotropy. However taking systematic uncertainties in the abundance
estimates into account, there is overall concordance in the range \eta = (3.4 -
6.9) x 10^{-10} @ 95% c.l. corresponding to a cosmological baryon density
\Omega_B h^2 = 0.012 - 0.025. If the above discrepancy is due to a neutrino
chemical potential, then upto 7.1 effective neutrino species are allowed by
nucleosynthesis. Other constraints on new physics are briefly discussed.
