High Energy Emission from Agn and Unified Schemes

Active Galactic Nuclei (AGN) are now known to be strong γ-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 flatspectrum 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. 1 Active Galactic Nuclei Active Galactic Nuclei (AGN) are extragalactic sources, in many cases clearly associated with nuclei of galaxies (although in the most distant objects the host galaxy is too faint to be seen), whose emission is dominated by non-stellar processes in some waveband(s) (typically but not exclusively the optical). One important feature of AGN is Invited Review Talk at the Vulcano Workshop Frontier Objects in Astrophysics and Particle Physics, Vulcano, Italy, May 1998 Radio Loudness Optical Emission Line Properties Type 2 (Narrow Line) Type 1 (Broad Line) Type 0 (Unusual) Radio-quiet: Seyfert 2 Seyfert 1