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, adv ection-dominated accretion flows (ADAFs). We then discuss a potentially use ful 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 th e theoretical point of view. Finally, we will touch on the most important i ssue in future research; that is, magnetic field activity in accretion disk s.