Contents: (1) Introduction. (2) Inferences insensitive to detailed model: Some fiducial numbers. Processes in ultrahot thermal plasma. Cyclotron/synchrotron and inverse Compton cooling. (3) Radial accretion flows: Spherical accretion. (4) Black holes according to general relativity: The Kerr metric. Three astrophysically important relativistic effects. (5) Accretion flows with angular momentum: Origin of infalling matter. Thin disks. General structure of tori or thick disks. Radiation-supported tori. Ion-supported tori. (6) Jet formation. (7) Some comments on phenomenology: The continuum spectrum. Ordinary QSOs. Radio galaxies. Radio quasars and optically violent variables (OVVs). Hard X-ray and gamma-ray sources. (8) Demography of AGNs. (9) Some concluding comments.