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