2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/13925This paper summarizes the observations which provide the best evidence for the presence of black holes in active galactic nuclei. This includes: X-ray variability; kinematical studies using optical emission lines as well as the distribution of megamaser spots; and the shape of the Fe K$α$ X-ray emission line. It also presents the current status of our understanding of jet-dominated active galaxies (blazars), and briefly reviews the currently popular AGN ``Unification Schemes.''To appear in ``Theory of Black Hole Accretion Disks'' (Cambridge University Press), 21 pages, 9 figuresAstrophysicstext