Relativistic jets from Active Galactic Nuclei (AGN) are powerful sources of radiation at every wavelength, but particularly in gamma-rays. Multi-wavelength observations in the last decade, driven by the new generation of large-area GeV and TeV telescopes, have revealed several new aspects of their structure and emission properties. This paper summarizes some recent progress in our understanding of relativistic jets, focusing in particular on the following results: a) evidence against the Broad Line Region (BLR) as origin of seed photons for the inverse Compton (IC) mechanism; b) spectral variability in gamma-rays that defies the usual SED classification, c) discovery of extreme-TeV sources, which challenge standard acceleration and emission scenarios, d) evidence of relativistic motion at the VLBI scale also in high-energy peaked BL Lacs. Other fundamental topics such as ultra-fast variability, neutrino emission and M87 images at event horizon scale are briefly mentioned, as discussed in other contributions at these proceedings.