  The search for periodic signals from blazars has become an actively pursued
field of research in recent years. This is because periodic signals bring us
information about the processes occurring near the innermost regions of
blazars, which are mostly inaccessible to our direct view. Such signals provide
insights into some of the extreme conditions that take place in the vicinity of
supermassive black holes that lead to the launch of the relativistic jets. In
addition, studies of characteristic timescales in blazar light curves shed
light on some of the challenging issues in blazar physics that includes
disk-jet connection, strong gravity near fast rotating supermassive black holes
and release of gravitational waves from binary supermassive black hole systems.
However, a number of issues associated with the search quasi-periodic
oscillations (QPOs) in blazar e.g., red-noise dominance, modest significance of
the detection, periodic modulation lasting for only a couple of cycles and
their transient nature, make it difficult to estimate the true significance of
the detection. Consequently, it also becomes difficult to make meaningful
inferences about the nature of the on-going processes. In this proceedings,
results of study focused on searching QPOs in a number of blazar
multi-frequency light curves are summarized. The time series analyses of long
term observations of the blazars revealed presence of year-timescale QPOs in
the sources including OJ 287 (optical), Mrk 501 (gamma-ray), J1043+2408 (radio)
and PKS 0219-164 (radio). As likely explanations, we discuss a number of
scenarios including binary supermassive black hole systems, Lense-Thirring
precession, and jet precession.
