  Despite their different nature and physics, blazars and gamma-ray bursts have
in common very powerful relativistic jets, which make them the most luminous
sources in the Universe. The energy extraction from the central compact object,
the jet collimation, the role and geometry of the magnetic fields, the
structure of the jet itself represent still big enough questions, that a
complete paradigm cannot yet be drawn. This article is concerned with the main
observational facts about blazars and gamma-ray burst jets, based on
multi-wavelength campaigns, and on the clues one can glean from these on jet
formation, behavior and powering. The future generation of telescopes and
instruments and the contributions from multi-messenger investigation
(astroparticle diagnostics and gravitational waves) will warrant further
significant progress.
