  We discuss the relevant processes for the relativistic electrons in the ICM
and the possible mechanisms responsible for the production of these electrons.
We focus on the origin of the radio halos giving some of the observational
diagnostics which may help in discriminating among the different models
proposed so far. Finally, we briefly discuss the discrepancy between the value
of the magnetic field assuming an inverse Compton (IC) origin of the hard X-ray
emission (HXR) and that obtained from Faraday Rotation Measurements (RM).
