  Relativistic jets are one of the most powerful manifestations of the release
of energy related to the 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. Recent high resolution radio
observations of flaring objects locate the high-energy emitting region
downstream the jet at parsec scale distance from the central engine, posing
questions on the nature of the seed photons upscattered to gamma-rays.
Furthermore, monitoring campaigns of the most active blazars indicate that not
all the high energy flares have the same characteristics in the various energy
bands, even from the same source, making the interpretation of the mechanism
responsible for the high-energy emission not trivial. Although the variability
of the most luminous blazars is well explained by the "shock-in-jet" scenario,
the sub-class of TeV emitting objects suggests a more complex emission model
with velocity gradients in a structured jet. This contribution presents results
obtained by recent multiwavelength campaigns of blazars aimed at studying the
radio and gamma-ray connection and the physical mechanisms at the basis of the
emission in these low and high energy bands.
