An active galactic nucleus (AGN) is a supermassive black hole residing at the core of a galaxy that is accreting material. The accretion occurs in the vicinity of the hot torus around the nucleus, and it can generate rapidly moving jets of charged particles that emit bright, variable radiation as material ccelertes as it falls inward. Quasars are perhaps the best-known luminous AGN, and their nuclei are relatively unobscured by dust. Quasar nuclear regions and disks are too far away and much too small to be resolved with telescopes and astronomers trying to understand the behavior of quasars, AGN, and accretion disks are forced to infer the physics from indirect measurements. Flux variability measurements offer one such avenue.