ABSTRACT
By any definition, AGN are extraordinary objects. Their extremely high luminosity, their spectacular images, their rapid and random time variability, their energy spectra, which belie thermal processes and indicate the presence of gigantic particle accelerators, mark them as some of the most important denizens of the cosmic zoo. The presence of a supermassive black hole, an accretion disk, a dusty torus, emission line clouds, a relativistic jet-these all indicate extraordinary complexity and a wonderful laboratory for astrophysical research. Before the advent of gamma-ray astronomy as a truly observational science, the basic phenomenon of AGN was known. It was the optical study of flat-spectrum radio sources (FSRS) that first drew attention to AGN since it was seen that the intense non-thermal radiation outshone the normal stellar population by many orders of magnitude. The observation of hot accretion disks, broad emission line clouds, and polarized radiation from the radio bands through x-rays heightened the interest.
