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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 the fact that their emission covers the whole electromagnetic spectrum, from the radio to the gamma-ray band, sometimes over almost 20 orders of magnitude in frequency.

It is now well established that AGN are strong gamma-ray (E > 100 MeV) emitters. To be more specific: 1. at least 40% of all EGRET sources are AGN (Thompson et al. 1995, 1996; some more AGN are certainly present amongst the still unidentified sources) and these make up almost 100% of all extragalactic sources (the only exceptions being the Large Magellanic Cloud and possibly Centaurus A); 2. all detected AGN are blazars, that is BL Lacertae objects (BL Lacs) or flat-spectrum radio quasars (FSRQ). (1) To appreciate the relevance of the latter point, we will first have to tackle the subject of AGN classification.

1 The term ``blazar'' is here given a wider meaning than the one sometimes implied, which is restricted to highly polarized quasars (HPQ) and/or optically violently variable (OVV) quasars. The reason is that there is increasing evidence that these categories and the flat-spectrum radio quasars, which reflect different empirical definitions, refer to more or less the same class of sources. That is, the majority of flat-spectrum radio quasars tend to show rapid variability and high polarization.