  The variability of active galactic nuclei (AGN) has long held the promise of
shedding light on their detailed structure, and possibly other astrophysical
phenonema. Different emission mechanisms lead to different patterns of
variability in flux which are in principle easily distinguishable. Recent
predictions for the expected spectrum of variations for various models are now
in such a form that they can be compared with the observed statistical
properties of AGN light curves from large scale monitoring programmes. In this
paper, we use the results of a long term monitoring programme of a large sample
of quasars and Seyfert galaxies, as well as individual light curves from the
literature, to distinguish between the various model predictions. The results
favour a model based on accretion disc instability over the starburst model
where the variation comes from a succession of supernova bursts, but it also
appears that much of the observed variation in quasars is due to gravitational
microlensing.
