  The number of diffuse radio halos 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. A model for the formation and maintenance of these
sources is suggested, including a first phase of relativistic particle
injection by past major merger events, starbust and AGN activity and a second
phase of reacceleration by the energy supplied from shocks and turbulence in a
recent merger event.
