  Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) are
two major pillars of cosmology. Standard BBN accurately predicts the primordial
light element abundances ($^4$He, D, $^3$He and $^7$Li), depending on one
parameter, the baryon density. Light element observations are used as a
baryometers. The CMB anisotropies also contain information about the content of
the universe which allows an important consistency check on the Big Bang model.
In addition CMB observations now have sufficient accuracy to not only determine
the total baryon density, but also resolve its principal constituents, H and
$^4$He. We present a global analysis of all recent CMB data, with special
emphasis on the concordance with BBN theory and light element observations. We
find $\Omega_{B}h^{2}=0.025+0.0019-0.0026$ and $Y_{p}=0.250+0.010-0.014$
(fraction of baryon mass as $^4$He) using CMB data alone, in agreement with
$^4$He abundance observations. With this concordance established we show that
the inclusion of BBN theory priors significantly reduces the volume of
parameter space. In this case, we find $\Omega_{B}h^2=0.0244+0.00137-0.00284$
and $Y_p = 0.2493+0.0006-0.001$. We also find that the inclusion of deuterium
abundance observations reduces the $Y_p$ and $\Omega_{B}h^2$ ranges by a factor
of $\sim $2. Further light element observations and CMB anisotropy experiments
will refine this concordance and sharpen BBN and the CMB as tools for precision
cosmology.
