The study of multiwavelength variability of active galactic nuclei (AGNs) is now a major branch of the field, enabled largely by the availability of suitable facilities for long-term studies of faint sources at many wavelengths; some of the scientific arguments for development of these facilities have been based on exactly these programs, which are now leading to improved understanding of the AGN phenomenon. It is only within the last few years that the evidence for supermassive black holes in both active and non-active galaxies has gone from circumstantial to compelling, and the potentially most powerful technique for measuring black-hole masses in AGNs is through study of broad emission-line variability. The existence of accretion disks in AGNs is still far from proven, but the evidence for them is improving, again as a result of variability studies. Multiwavelength monitoring observations are beginning to show the relationships between variability in different bands, and the hope is that once the phenomenology is better known, our understanding of the physics will follow.
In this contribution, we will cover the basic characteristics of AGN variability and provide what we hope is some relevant historical background. Because of the current importance of emission-line variability studies, we will develop the theory of reverberation mapping from first principles. One of the most powerful and widely used tools in the analysis of emission-line and continuum variability data is the technique of cross-correlation, and we will therefore describe in some detail the application of this method to AGN data. Throughout this contribution, we will concentrate almost exclusively on non-blazar AGNs, those for which we believe most of the observed UV/optical emission originates in an accretion disk rather than in a relativistic jet: many of the techniques described here are also applicable to blazars, however. For a fairly recent discussion of blazar variability results, we refer the reader to the fine review by Ulrich et al. 86. We emphasize that we intend for this contribution to be primarily instructional; this should not be misconstrued as a comprehensive review of the state of the field. Our intent is to provide both students and researchers who already have some familiarity with AGNs with enough background to read critically the current literature on AGN variability and understand the strengths and weaknesses of the method.