  Studying simultaneous optical and X-ray light curves of radio-quiet AGN can
help to probe the relationship between very different physical components - the
cool, optically thick disk and hot, optically thin corona. Here, we review the
relationship between optical and X-ray variability in Seyfert galaxies, which
due to observing constraints was difficult to study for many years, but was
given a huge boost with the launch of the RXTE satellite in 1995. We summarise
the diverse results of several monitoring campaigns, which pose a challenge for
standard theories relating optical and X-ray variability, with sources showing
either correlated optical and X-ray flux variations, correlated optical flux
and X-ray spectral variations, or no correlation at all. We discuss possible
explanations for these results, some of which may be explained using a more
standard AGN picture, while others may require additional components, such as
the 2-phase accretion flows suggested to explain black hole X-ray binary
behaviour.
