  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 continuum
variability of radio-quiet AGN and the relation between the two bands. We
demonstrate the strong connection between the X-ray variability properties of
AGN and the variability of stellar-mass black hole candidates on much shorter
time-scales, and discuss the implications of this result for the origins of the
variability. The relationship between optical and X-ray variability is complex,
with some AGN showing strong X-ray/optical correlations while others show no
obvious correlation. We suggest a possible explanation for this variety of
behaviour.
