  Optical identification of hard X-ray selected BeppoSAX and Chandra sources
indicates that a large fraction of the sources are ``intermediate'' AGN, i.e.
type 1.8-1.9 AGN, broad-line quasars and even X-ray loud but optically dull
(apparently normal) galaxies, all obscured in X-rays by columns of the order of
logNH=22-23.5 cm-2. Because of this obscuring matter, these sources are more
difficult to detect or select at other wavelengths, implying that a fraction of
the accretion power in the Universe may have been missed so far.
