  Multiwavelength observations of the hard X-ray selected sources discovered by
BeppoSAX, Chandra and XMM-Newton surveys have significantly improved our
knowledge of the AGN population. The increasing number of X-ray obscured AGN so
far discovered confirms the prediction of those AGN synthesis models for the
X-ray background based on the Unified scheme. However, follow-up optical
observations of hard X-ray selected sources indicate that their optical
properties are quite varied and the simple relations between optical and X-ray
absorption are by no means without exception. Moreover, there is evidence of a
substantial number of luminous X-ray sources hosted by apparently normal
galaxies. In this paper the results obtained from multiwavelength observations
of hard X-ray selected sources discovered by BeppoSAX and XMM-Newton are
presented and briefly discussed.
