heavily absorbed and about 80% of the light produced by accretion will be absorbed by gas and dust which may reside in nuclear starburst regions that feed the AGN [@fabian1998]. However, these models are far from unique and contain a number of often overlooked assumptions, so their predictive power remains limited until complete samples of spectroscopically classified hard X-ray sources are available. In particular, they require a substantial contribution of high-luminosity absorbed X-ray sources (type-2 QSOs), which so far have only scarcely been detected. The cosmic history of obscuration and its potential dependence on intrinsic source luminosity remain completely unknown. @gilli2001 e.g. assumed a strong evolution of the absorbed/obscured fraction (ratio of type-2/type-1 AGN) from 4:1 in the local universe to much larger fractions (10:1) at high redshifts [see also @fabian1998]. The gas-to-dust ratio in high-redshift, high-luminosity AGN could be completely different from the usually assumed Galactic value due to sputtering of the dust particles in the strong radiation field [@granato1997]. There could thus be objects which are heavily absorbed at X-rays and unobscured at optical wavelengths.

After having understood the basic contributions to the X-ray background, the general interest is now focussing on understanding the physical nature of these 