  Magnetic fields in galaxy clusters have been measured using a variety of
techniques, including: studies of synchrotron relic and halo radio sources
within clusters, studies of inverse Compton X-ray emission from clusters,
surveys of Faraday rotation measures of polarized radio sources both within and
behind clusters, and studies of Cluster Cold Fronts in X-ray images. These
measurements imply that most cluster atmospheres are substantially magnetized,
with typical field strengths of order 1 microGauss with high areal filling
factors out to Mpc radii. There is likely, however, to be considerable
variation in field strengths and topologies both within and between clusters,
especially when comparing dynamically relaxed clusters to those that have
recently undergone a merger. In some locations, such as the cores of cooling
flow clusters, the magnetic fields reach levels of 10 - 40 microGauss, and may
be dynamically important. In all clusters the magnetic fields have a
significant effect on energy transport in the intracluster medium. We also
review current theories on the origin of cluster magnetic fields.
