  The existence of magnetic fields associated with the intracluster medium in
clusters of galaxies is now well established through different methods of
analysis. Magnetic fields are investigated in the radio band from studies of
the rotation measure of polarized radio galaxies and the synchrotron emission
of cluster-wide diffuse sources. Other techniques include X-ray studies of the
inverse Compton emission and of cold fronts and magneto hydrodynamic
simulations. We review the main issues that have led to our knowledge on
magnetic fields in clusters of galaxies. Observations show that cluster fields
are at the $\mu$G level, with values up to tens of $\mu$G at the center of
cooling core clusters. Estimates obtained from different observational
approaches may differ by about an order of magnitude. However, the discrepancy
may be alleviated by considering that the magnetic field is not constant
through the cluster, and shows a complex structure. In particular, the magnetic
field intensity declines with the cluster radius with a rough dependence on the
thermal gas density. Moreover, cluster magnetic fields are likely to fluctuate
over a wide range of spatial scales with values from a few kpc up to hundreds
kpc. Important information on the cluster field are obtained by comparing the
observational results with the prediction from numerical simulations. The
origin of cluster magnetic fields is still debated. They might originate in the
early Universe, either before or after the recombination, or they could have
been deposited in the intracluster medium by normal galaxies, starburst
galaxies, or AGN. In either case, magnetic fields undergo significant
amplification during the cluster merger processes.
