  The spectrum of the hard X-ray background records the history of accretion
processes integrated over the cosmic time. Several observational and
theoretical evidences indicate that a significant fraction of the energy
density is obscured by large columns of gas and dust. X-ray surveys are the
most efficient way to trace accretion onto supermassive black holes, since
obscured, accreting sources are more difficult to select at all other
wavelengths. The current status of hard X-ray surveys after the recent
observations carried out with Chandra and XMM-Newton satellites is reviewed
along with the results of extensive follow-up multiwavelength observations. In
particular recent results concerning the physical and evolutive properties of
the supermassive black holes hosted by X-ray selected Active Galactic Nuclei at
high redshifts will be discussed.
