EXOSAT observations of variability in active galactic nuclei.

Exosat observations of variability in Seyfert galaxies and quasars are reviewed. The range of variability considered extends from short time scales of a few hundred seconds to times scales of 10 to the 7th seconds. The different types of behavior which are characterized by the power spectra of different power law slopes are examined. The power spectra are compared with statistical models. Also the short medium-energy AGN observations are compared with 'long look' for NGC5506. The results suggest that there is no characteristic time scale associated with light-travel time or dynamical time scale around a massive black hole.