  The main component of the intracluster medium (ICM) in clusters of galaxies
is represented by the X-ray emitting thermal plasma. In addition, the presence
of relativistic electrons and large-scale magnetic fields in a fraction of
galaxy clusters is demonstrated by the detection of large-scale synchrotron
radio sources, which have no optical counterpart and no obvious connection to
the cluster galaxies. Observational results provide evidence that these
phenomena are related to cluster merger activity, which supplies the energy for
the reacceleration of the radio emitting particles. The investigation of the
halo-merger connection is of great importance to the knowledge of the formation
and evolution of clusters of galaxies.
