  The presence of a helical magnetic field threading the jet of an Active
Galactic Nucleus (AGN) should give rise to a gradient in the observed Faraday
rotation measure (RM) across the jet, due to the associated systematic change
in the line-of-sight magnetic field. Reports of observations of transverse RM
gradients across AGN jets have appeared in the literature starting from 2002,
but concerns were raised about the resolution required for these gradients to
be reliable, and there was a lack of a full understanding of the best approach
to accurate estimation of the uncertainties of local RM values. These questions
have now been resolved by recent Monte Carlo simulations carried out by various
groups, enabling both a verification of previously published results and
reliable analyses of new data. We consider here RM gradients across the jet
structures of 15 AGN, some previously published in the refereed literature but
without a correct and complete error analysis, and some published for the first
time here, all of which have monotonic transverse RM gradients with
significances of at least 3 sigma.
