We investigate the role of disorder for field-driven
quantum phase transitions of metallic antiferromagnets. For systems
with sufficiently low symmetry, the combination of a uniform
external field and non-magnetic impurities leads effectively to a
random magnetic field which strongly modifies the behavior close to
the critical point. Using perturbative renormalization group, we
investigate in which regime of the phase diagram the disorder
affects critical properties. In heavy fermion systems where even
weak disorder can lead to strong fluctuations of the local Kondo
temperature, the random field effects are especially pronounced. We
study possible manifestation of random field effects in experiments
and discuss in this light neutron scattering results for the field
driven quantum phase transition in CeCu5.8Au0.2.