Clusters of galaxies are the largest aggregates of matter that have decoupled from the universal expansion and have approximately evolved to a proper dynamical equilibrium configuration. While in the optical they are just observed as dense concentrations of galaxies, they are seen in X-rays as continuously connected entities through the emission of the X-ray luminous intracluster plasma (e.g. Sarazin 1986). Relativistic particles and magnetic fields are observed in clusters in radio halos, in regions around radio galaxies and through Faraday rotation of the radiation from background radio sources (see contribution by L. Feretti and e.g. Kronberg, 1994). Thus Clusters of galaxies are the largest, well characterized astrophysical laboratories in which plasma physical processes of diffuse media can be studied. In this brief summary the bulk properties of the intracluster medium (ICM) are discussed and two studies of the interaction of galactic radio lobes with the intracluster plasma are presented (related topics are discussed by F. Owen and L. Feretti in this volume).