  In this chapter we discuss the X-ray radiation from relativistic accretion
disks around supermassive black holes, supposed to exist in the centers of
Active Galactic Nuclei (AGN). Our focus is on the X-ray radiation, especially
in the Fe K$\alpha$ line which originates in the innermost parts of an
accretion disk. Moreover, here we discuss some effects which can disturb the Fe
K$\alpha$ profile and cause its rapid and irregular variability, observed in
the X-ray spectra of some AGN. We will pay attention to three such effects:
perturbations in the disk emissivity, absorbtion by warm absorbers and
gravitational microlensing. The X-ray emission from accretion disks around
non-rotating (Schwarzschild metric), as well as rotating (Kerr metric)
supermassive black holes, is discussed. The X-ray radiation of AGN is probably
produced in a compact region near their central supermassive black holes, and
can provide us some essential information about the plasma conditions and the
space-time geometry in these regions. The goal of this chapter is mainly to
present a short overview of some important and recent investigations in this
field.
