  We discuss what can be learned about the baryon density from an all-sky map
of the cosmic microwave background (CMB) with sub-degree angular resolution.
With only minimal assumptions about the primordial spectrum of density
perturbations and the values of other cosmological parameters, such a CMB map
should be able to distinguish between a Universe with a baryon density near 0.1
and a baryon-dominated Universe. With additional reasonable assumptions, it is
conceivable that such measurements will constrain the baryon density to an
accuracy similar to that obtained from BBN calculations.
