Clusters of Galaxies: Multifrequency Observations versus Theories

Clusters of galaxies are large gravitationally bound systems which consist of several observable components: hundreds of galaxies, hot gas between the galaxies and sometimes relativistic particles. These components are emitting in different wavelengths from radio to X-rays. We show that the combination of observations at different frequencies and also theoretical models is giving now a comprehensive picture of these massive objects. Topics presented here include cluster masses, baryon fractions, the dynamical state of clusters, the physical processes in clusters and cosmological parameters derived from cluster observations.