  While the major contribution to the Cosmic Microwave Background (CMB)
anisotropies are the sought-after primordial fluctuations produced at the
surface of last scattering, other effects produce secondary fluctuations at
lower redshifts. These secondary fluctuations must be carefully accounted for,
in order to isolate primordial fluctuations. In addition, they are interesting
in their own right, since they provide a wealth of information on the geometry
and local properties of the universe. Here, I survey the different sources of
secondary anisotropies and extragalactic foregrounds of the CMB. I show their
relative importance on the multipole-frequency plane. I discuss in particular
their impact in the future CMB missions MAP and Planck Surveyor.
