  Deep X-ray surveys have shown that the cosmic X-ray background (XRB) is
largely due to the accretion onto supermassive black holes, integrated over the
cosmic time. These surveys have resolved more than 80% of the 0.1-10 keV X-ray
background into discrete sources. Optical spectroscopic identifications show
that the sources producing the bulk of the X-ray background are a mixture of
obscured (type-1) and unobscured (type-2) AGNs, as predicted by the XRB
population synthesis models. A class of highly luminous type-2 AGN, so called
QSO-2s, has been detected in the deepest Chandra and XMM-Newton surveys. The
new Chandra AGN redshift distribution peaks at much lower redshifts (z~0.7)
than that based on ROSAT data, indicating that Seyfert galaxies peak at
significantly lower redshifts than QSOs.
