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. 
In this talk I present recent results on multiwavelength campaigns of
blazars at low and high redshift detected by Fermi-LAT during a
flaring activity. The high resolution of
the radio VLBI observations allowed a deep look into the innermost
region of the AGN, providing important clues on the jet
structure, as well as on the flux density and polarization
variability. These results will help us in better understanding the
physics of relativistic jets.