XRB flux has been resolved using 1.4 Msec observations with [*ROSAT*]{} [@hasinger1998] and 1-2 Msec Chandra observations [@brandt2001a; @rosati2002; @brandt2002] and 100 ksec observations with XMM-Newton [@hasinger2001]. In the harder (2-10 keV) band a similar fraction of the background has been resolved with the above Chandra and XMM-Newton surveys, reaching source densities of about 4000 deg$^{-2}$. Surveys in the very hard (5-10 keV) band have been pioneered using BeppoSAX, which resolved about 30% of the XRB [@fiore1999]. XMM-Newton and Chandra have now also resolved the majority (60-70%) of the very hard X-ray background.

Optical follow-up programs with 8-10m telescopes have been completed for the [*ROSAT*]{} deep surveys and find predominantly Active Galactic Nuclei (AGN) as counterparts of the faint X-ray source population [@schmidt1998; @zamorani1999; @lehmann2001], mainly X-ray and optically unobscured AGN (type-1 Seyferts and QSOs) and a smaller fraction of obscured AGN (type-2 Seyferts). The X-ray observations have so far been about consistent with population synthesis models based on unified AGN schemes [@comastri1995; @gilli2001], which explain the hard spectrum of the X-ray background by a mixture of X-ray absorbed and unabsorbed AGN, folded with the corresponding luminosity function and its cosmological evolution. According to these models, most AGN spectra are 