Clusters of galaxies: a fundamental pillar of cosmology

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.