  Clusters of galaxies are used in a variety of ways to do cosmology. Some of
them are presented here. Their X-ray emitting gas allows us to determine the
baryon fraction, dark matter distribution and the matter density $\Omega_{m}$
of the universe. Another interesting component is relativistic particles whose
radio emission provide the measure of the magnetic fields ($\approx \mu G$) in
the intra-cluster medium (ICM). The observation of distant clusters of galaxies
is also important for cosmology. In particular the new X-ray satellites CHANDRA
and XMM yield exciting new results for galaxy cluster physics and cosmology.
