  We first present a short overview of X-ray probes of the black hole region of
active galaxies (AGN) and then concentrate on the X-ray search for supermassive
black holes (SMBHs) in optically non-active galaxies.
  The first part focuses on recent results from the X-ray observatories Chandra
and XMM-Newton which detected a wealth of new spectral features which originate
in the nuclear region of AGN.
  In the last few years, giant-amplitude, non-recurrent X-ray flares have been
observed from several non-active galaxies. All of them share similar
properties, namely: extreme X-ray softness in outburst, huge peak luminosity
(up to ~10^{44} erg/s), and the absence of optical signs of Seyfert activity.
Tidal disruption of a star by a supermassive black hole is the favored
explanation of these unusual events. The second part provides a review of the
initial X-ray observations, follow-up studies,and the relevant aspects of tidal
disruption models studied in the literature.
