  Planck will be the first mission to map the entire cosmic microwave
background (CMB) sky with mJy sensitivity and resolution better than 10'. The
science enabled by such a mission spans many areas of astrophysics and
cosmology. In particular it will lead to a revolution in our understanding of
primary and secondary CMB anisotropies, the constraints on many key
cosmological parameters will be improved by almost an order of magnitude (to
sub-percent levels) and the shape and amplitude of the mass power spectrum at
high redshift will be tightly constrained.
