Variability is ubiquitous in active galactic nuclei (AGN) and can be observed or inferred at all timescales, from hours to billions of years. In this talk I will first illustrate how spatially resolved optical spectroscopy, combined with high quality X-ray data, can be used to probe AGN variability on > 10^4 yr timescales. I will then present a compilation of variability measurements, covering many orders of magnitude both in time lags and variability amplitude, which provides an overview of the variabiltiy phenomena. This compilation includes ensemble studies, which characterise the mean variability among the AGN population, as well as measurements for single objects such as the changing look AGN, the Voorwerpjes galaxies, and our own Sagittarius A*. Finally, I will present a framework which allows us to test if and how variability in different AGN and at different timescales can be linked and explained based on the distribution of the Eddington ratio (ER) among the galaxy population. At the end of the talk I will also discuss possible applications of our model, e.g. for understanding extreme variable AGN or planning future time domain surveys such as LSST.