Several active galactic nuclei (AGN) show UV/optical variability lagging behind the X-ray emission by a few days. The simplest and most straightforward interpretation is that the variable X-ray flux from the corona illuminates the accretion disc below where it is partially reflected and observed as a fast X-ray reverberation signal, and partially absorbed and thermalised in the disc, which produces a slow UV and optical reverberation signal. Since the corona is thought to be centrally located and very small compared to the accretion disc, it first illuminates the hottest inner parts of the accretion disc and then illuminates the colder outer areas. Thus, one expects to see the original X-ray fluctuations to be firstly followed by variations in the UV and then in the optical.
In this talk, I will introduce the concept of reverberation and how it can be used to map the different regions of AGN. I will also present the current observational and modelling efforts to explain the broadband X-ray/UV/optical spectral and timing behavior of AGN. I will also discuss further implications of thermal reverberation on AGN physics, and how it could act as a synergy with X-ray observations to probe the innermost regions of AGN.