Affiliated to the Astrophysics Division, Space Science Department, European Space Agency
On leave from Dipartimento di Fisica, II Università di Roma, Italy
ABSTRACT. Active Galactic Nuclei (AGN) are now known to be strong -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.
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