  The number of known diffuse radio sources in clusters of galaxies has grown
in recent years, making it possible to derive statistical properties of these
sources and of the hosting clusters. We show that diffuse sources are
associated with X-ray luminous clusters which have undergone recent merger
processes. The radio and X-ray structures are often similar, and correlations
are found between radio and X-ray parameters. This is indication of a link
between the diffuse relativistic and thermal plasma in clusters of galaxies.
