Some of the more efficient accretion processes in astrophysics are associated with the presence of accretion disks. Such disks are found in protostars; in various types of binary stars, including low- and high-mass X-ray binaries; in dwarf novae or cataclysmic variables; and in classical novae. They are also believed to be present in galactic nuclei, around the central supermassive BH, during periods of fast accretion.

Accretion disks in galactic centers are naturally formed by infalling gas that sinks into the central plane of the galaxy while retaining most of its angular momentum. The assumption is that the viscosity in the disk is sufficient to provide the necessary mechanism to transfer outward the angular momentum of the gas and to allow it to spiral into the center, losing a considerable fraction of its gravitational energy on the way. The energy lost in the process can be converted into electromagnetic radiation with extremely high efficiency, from about 4 percent and up to 42 percent. It can also be converted to kinetic energy of gas, which is blown away from the disk, or in other cases, it can heat the gas to very high temperatures, which causes much of the energy to be advected into the BH.

AGN disks, and accretion disks in general, are classified according to their shape into thin, slim, and thick disks. Each one of these can be optically thin or thick, depending on the column density (or surface density) and the level of ionization of the gas. The optical depth of AGN disks, during periods of fast accretion, is very large. The disks that receive most attention are optically thick, geometrically thin accretion disks. Such systems are easier to treat analytically and numerically. The next section describes the basic assumptions and the analytical solution of such disks. A full solution of this type can be used to calculate the emergent disk spectrum and to compare it with observations. The additional sections address other types of accretion disks and accretion flows.