Abstract: I will briefly overview the main observational properties of AGN variability, including the (timescale-dependent) bluer-when-brighter trend in UV/optical. This newly discovered timescale-dependent behavior directly rules out the host contamination as well as the change of global accretion rate as the primary contribution to the UV/optical variability, but is naturally favored by the thermally inhomogeneous accretion disk scenario. Furthermore, this inhomogeneous accretion disk model has been tentatively developed to be able to produce the UV/optical continuum lag in AGN without light echoing (or X-ray reprocessing scenario).