Abstract

Matter accretes on the black hole through the disc powering the relativistic jet, which dominates the emission at all wavelengths in blazars. The exact mechanisms responsible for the origin, acceleration and collimation of the jet are not well understood. Since central engine is not resolvable by any existing optical facility, variability in flux can be used as a tool to understand the structure and physical processes in the jet. Blazars show variability in their flux at all frequencies and polarization at radio and optical with time scales ranging from years to minutes and hence are good candidates for such study. Long-term continuous monitoring in all energy bands with high temporal resolution is the key to probe size of emission regions, emission processes and structure of the jet. Some details about the present understanding and the role new facilities might play will be discussed, along with some results on the variability of blazars, in particular, S5 0716+714.