  CMB foregrounds consist of all radiation between the surface of last
scattering and the detectors, which can interfere with the cosmological
interpretation of CMB data. Fortunately, in temperature (intensity), even
though the foregrounds are complex they can relatively easily be mitigated.
However, in polarization, diffuse Galactic radiation (synchrotron and thermal
dust) can be polarized at a level of >10 % making it more of a challenge. In
particular, CMB B-modes, which are a smoking-gun signature of inflation, will
be dominated by foregrounds. Component separation will therefore be critical
for future CMB polarization missions, requiring many channels covering a wide
range of frequencies, to ensure that foreground modelling errors are minimised.
