  Most Active Galactic Nuclei (AGN) are `obscured', i.e. the nucleus is hiding
behind a screen of absorbing material. The advantage of having the nucleus
obscured is to make easier the observations of those emission components which
originate in circumnuclear matter outside the absorbing regions, because in
this case they are not outshined by the nuclear emission. This is particularly
important in X-rays, where spatial resolution is (with the notable exception of
Chandra) poorer than in the optical, and the study of circumnuclear regions is
often based on spectral analysis only.
  The properties of circumnuclear matter, in the light of recent high spectral
and/or angular resolution $Chandra$ and XMM--Newton observations, will be
reviewed in Sec.2. In Sec.3 we will discuss obscured AGN in the framework of
the Unification Model. Recent discoveries of X-ray obscured Seyfert 1, and of
X-ray loud but optically normal galaxies, are calling for a revision of the
Unification Model.
  Obscured AGN have also a cosmological relevance. Not only are they a
fundamental ingredient of synthesis models of the Cosmic X-ray Background
(XRB), but provide a link between the XRB and the Cosmic Infrared Background,
as briefly discussed in Sec.4.
