Active galactic nuclei

Our current knowledge of active galactic nuclei is reviewed. The importance of observational data taken over a wide range of frequencies, from radio and infrared through optical and ultraviolet to X-rays and gamma -rays, is emphasized. Important overall principles include the continuity from quasars and QSOs through Seyfert and radiogalaxies to low-luminosity LINERs, the importance of considering roughly cylindrically symmetric (rather than spherically symmetric) structures, and that the various regions generally have different axes and planes of symmetry, and are often warped.