One of the main challenges facing upcoming cosmic microwave background (CMB
) experiments will be to distinguish the cosmological signal from foregroun
d contamination. We present a comprehensive treatment of this problem and s
tudy how foregrounds degrade the accuracy with which the Boomerang, MAP, an
d Planck experiments can measure cosmological parameters. Our foreground mo
del includes not only the normalization, frequency dependence, and scale de
pendence for each physical component, but also variations in frequency depe
ndence across the sky. When estimating how accurately cosmological paramete
rs can be measured, we include the important complication that foreground m
odel parameters (we use about 500) must be simultaneously measured from the
data as well. Our results are quite encouraging: despite all these complic
ations, precision measurements of most cosmological parameters are degraded
by less than a factor of 2 for our main foreground model and by less than
a factor of 5 in our most pessimistic scenario. Parameters measured though
large-angle polarization signals suffer more degradation: up to 5 in the ma
in model and 25 in the pessimistic case. The foregrounds that are potential
ly most damaging and therefore most in need of further study are vibrating
dust emission and point sources, especially those in the radio frequencies.
It is well known that E and B polarization contain valuable information ab
out reionization and gravity waves, respectively. However, the crosscorrela
tion between polarized and unpolarized foregrounds also deserves further st
udy, as we find that it carries the bulk of the polarization information ab
out most other cosmological parameters.