SIMBAD references

Although the variability of quasars played a crucial role in the initial understanding of active galactic nuclei, since then little progress has been made in understanding the nature of quasar variations. This is partly through lack of high quality data taken over sufficiently long periods of time, and partly lack of theoretical modeling of the processes taking place in the active nucleus in a form which would allow quantitative comparison with observations. In this paper we make the case that variability can provide an essential clue as to the structure and mechanism of the emitting region, and review what is known about AGN variations in optical passbands, especially on longer timescales where most of the power resides. We also use theoretical predictions from the literature for various models of variation and compare them with observations. Although the models are still in preliminary stages, it is already clear that observations have the power to discriminate between them. Another aspect of AGN variability is the possibility that quasars are being gravitationally microlensed. This has been well-established in multiply lensed quasar systems, and may well take place in most quasars. Where this can be convincingly established it can be used to constrain the structure of the accretion disk and throw light on the nature of the microlensing bodies. Finally, we make some remarks regarding what is needed for future observations, and the need for model predictions which can be compared with observations in such a way that they can be eliminated, or their parameters refined.