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Long-time-scale X-ray and optical variability is a key characteristic of AGN.Here, we summarise our current understanding of the X-ray and optical continuumvariability of radio-quiet AGN and the relation between the two bands. Wedemonstrate the strong connection between the X-ray variability properties ofAGN and the variability of stellar-mass black hole candidates on much shortertime-scales, and discuss the implications of this result for the origins of thevariability. The relationship between optical and X-ray variability is complex,with some AGN showing strong X-ray/optical correlations while others show noobvious correlation. We suggest a possible explanation for this variety ofbehaviour.Comment: 8 pages, invited contribution to the conference 'Stellar mass,  Intermediate-mass and Supermassive black holes', October 28-31 2003, Kyoto,  Japan (proceedings to be published in Progress of Theoretical Physics

A Brief Review of Long-Term X-Ray and Optical Variability in Radio-Quiet AGN