  Relativistic jets are one of the most powerful manifestations of the release
of energy produced around supermassive black holes at the centre of active
galactic nuclei (AGN). Their emission is observed across the entire
electromagnetic spectrum, from the radio band to gamma rays. Despite decades of
efforts, many aspects of the physics of relativistic jets remain elusive. In
particular, the location and the mechanisms responsible for the high-energy
emission and the connection of the variability at different wavelengths are
among the greatest challenges in the study of AGN. From the comparison of the
radio and gamma-ray light curves of gamma-ray flaring objects, there is
evidence that some flares, either in radio or in gamma rays, have not an
obvious connection at the other extreme of the electromagnetic spectrum, like
in the case of the Narrow-Line Seyfert 1 SBS 0846+513. An intriguing aspect
pointed out by high resolution radio observations is the change of the
polarization properties close in time with some high energy flares. In
particular, in PKS 1510-089 and 3C 454.3 a rotation of almost 90 degrees has
been observed after strong gamma-ray flares. The swing of the polarization
angle may be related either to the propagation of a shock along the jet that
orders the magnetic field, or a change of the opacity regime.
