  Active Galactic Nuclei (AGN) are now known to be strong gamma-ray emitters.
After briefly describing AGN classification and the main ideas behind unified
schemes, I summarize the main properties of blazars (that is BL Lacs and
flat-spectrum radio quasars) and their connection with relativistic beaming.
Finally, I address the question of why blazars, despite being extreme and very
rare objects, are the only AGN detected at high (E > 100 MeV) energies, and
touch upon the relevance of TeV astronomy for AGN research.
