  Cosmological simulations of magnetic fields in galaxy clusters show that
remarkable agreement between simulations and observations of Faraday rotation
and radio haloes can be achieved assuming that seed fields of 10^{-9} G were
present at redshifts 15-20. The structure of the seed field is irrelevant for
the final intracluster field. On average, the field grows exponentially with
decreasing cluster redshift, but merger events cause steep transient increases
in the field strength. Typical field-reversal scales are of order 50 kpc/h. In
most cases, the intracluster fields are dynamically unimportant. Assuming
secondary electron models, the average structure of cluster radio haloes can
naturally be reproduced.
