  Here we present an overview of some of the most significant observational and
theoretical studies of the broad Fe K{\alpha} spectral line, which is believed
to originate from the innermost regions of relativistic accretion disks around
central supermassive black holes of galaxies. The most important results of our
investigations in this field are also listed. All these investigations indicate
that the broad Fe K{\alpha} line is a powerful tool for studying the properties
of the supermassive black holes (such as their masses and spins), space-time
geometry (metric) in their vicinity, their accretion physics, probing the
effects of their strong gravitational fields, and for testing the certain
predictions of General Relativity.
