  The integrated emission of highly obscured AGN is expected to provide a major
contribution to the X-ray energy density in the Universe: the X-ray background
(XRB). The study of these objects is possible only at energies where the
effects of absorption are less severe. For this reason we have carried out the
BeppoSAX High Energy LLarge Area Survey in the hardest band (5-10 keV)
accessible so far with imaging X-ray instruments. The source surface density at
the survey limiting flux accounts for a significant fraction (20-30%) of the
hard XRB. The X-ray data complemented by multiwavelength follow--up
observations suggest that a large fraction of the hard sources are AGN and that
X-ray absorption with column densities in the range 10^22-23.5 cm-2 is common
among them. The great diversity in their optical-near-IR properties suggests
that the optical appearance of obscured sources is a function of the X-ray
luminosity. We briefly discuss the implications of these findings for the XRB
models.
