  The recent evidences of the presence of magnetic fields and relativistic
particles in galaxy clusters are reviewed. The existence of microG-level
magnetic fields in cluster atmospheres appears well established from the
detection of diffuse radio emission and from studies of Rotation Measure. The
fact that the diffuse radio emission is not common in clusters favours the
hypothesis that the population of relativistic particles, produced during the
cluster formation by AGNs or star formation, are reaccelerated by recent
cluster merger processes.

Astrophysics