Opening the time domain for Supermassive Black Hole studies across the Universe.

Timing studies have been one of the main tools by which we inferred the presence of Supermassive Black Holes (SMBH) in the center of Galaxies, and they provided crucial insights into the structure of accreting sources ($i.e.$, Active Galactic Nuclei). Variability studies have now expanded to a new dimension, with both large and deep surveys probing the time domain in order to obtain a census of SMBH in the Universe. I will describe some of the efforts done in the last two decades to detect and characterise AGN through variability both in the X-ray and in the optical, as well as the expectations from upcoming next generation surveys such as eRASS and LSST.