Black-Hole Accretion Disks

We overview the theory of black hole accretion disks. In the first half, we introduce basic accretion disk models with emphasis on optically thin, advection-dominated accretion flows (ADAFs). We then discuss a potentially useful test of the disk model, which uses gravitational microlens events. The second part will be on time variability and time-dependent models. We will summarize observed complex variability and discuss its implications from the theoretical point of view. Finally, we will touch on the most important issue in future research; that is, magnetic field activity in accretion disks.