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Deep X-ray surveys have shown that the cosmic X-ray background (XRB) islargely due to the accretion onto supermassive black holes, integrated over thecosmic time. These surveys have resolved more than 80% of the 0.1-10 keV X-raybackground into discrete sources. Optical spectroscopic identifications showthat the sources producing the bulk of the X-ray background are a mixture ofobscured (type-1) and unobscured (type-2) AGNs, as predicted by the XRBpopulation synthesis models. A class of highly luminous type-2 AGN, so calledQSO-2s, has been detected in the deepest Chandra and XMM-Newton surveys. Thenew Chandra AGN redshift distribution peaks at much lower redshifts (z~0.7)than that based on ROSAT data, indicating that Seyfert galaxies peak atsignificantly lower redshifts than QSOs.Comment: to appear in proceedings of the 13th Annual Astrophysics Conference  in Maryland: The Emergence of Cosmic Structure, Stephen S. Holt and Chris  Reynolds (eds

Formation and Evolution of Supermassive Black Holes in Galactic Centers: Observational Constraints