  Active galactic nuclei (AGN) are variable on a wide range of timescales,
though relatively few systematic variability surveys have been conducted.
Previous broad-band (both spectral and temporal) variability surveys of AGN are
limited in their temporal and spectral bandwidth, despite their promise for
probing the central engine and black hole mass. We outline optimal properties
for variability studies and provide a brief summary of three new variability
surveys, two (BATSS and DASCH) about to begin and the third (EXIST) possible
within the next decade, which will open new windows on the physics and
fundamental properties of AGN.
