  The most spectacular aspect of cluster radio emission is represented by the
large-scale diffuse radio sources, which cannot be obviously associated with
any individual galaxy. These sources demonstrate the existence of relativistic
particles and magnetic fields in the cluster volume, thus indicating the
presence of non-thermal processes in the hot intracluster medium. The knowledge
of the properties of these sources has increased significantly in recent years,
owing to sensitive radio images and to the development of theoretical models.
An important piece of information on the origin and evolution of these sources
can be obtained by the cluster X-ray emission of thermal origin, and by its
relation to the radio emission. Moreover, non-thermal X-ray emission of inverse
Compton origin gives direct information on the energy density of radio emitting
particles and the intensity of magnetic field.
