  Observations of the cosmic microwave background (CMB), especially of its
frequency spectrum and its anisotropies, both in temperature and in
polarization, have played a key role in the development of modern cosmology and
our understanding of the very early universe. We review the underlying physics
of the CMB and how the primordial temperature and polarization anisotropies
were imprinted. Possibilities for distinguishing competing cosmological models
are emphasized. The current status of CMB experiments and experimental
techniques with an emphasis toward future observations, particularly in
polarization, is reviewed. The physics of foreground emissions, especially of
polarized dust, is discussed in detail, since this area is likely to become
crucial for measurements of the B modes of the CMB polarization at ever greater
sensitivity.
