sources, the cosmological evolution of their properties, and their role in models of galaxy evolution. We know that basically every galaxy with a spheroidal component in the local universe has a supermassive black hole in its centre [@gebhardt2000]. The luminosity function of X-ray selected AGN shows strong cosmological density evolution at redshifts up to 2, which goes hand in hand with the cosmic star formation history [@miyaji2000]. At the redshift peak of optically selected QSOs, around $z$=2.5, the AGN space density is several hundred times higher than locally, which is in line with the assumption that most galaxies have been active in the past and that the feeding of their black holes is reflected in the X-ray background. While the comoving space density of optically and radio-selected QSOs has been shown to decline significantly beyond a redshift of 2.5 [@schmidt1997; @fan2001; @shaver1996], the statistical quality of X-ray selected high-redshift AGN samples still needs to be improved [@miyaji2000]. The new Chandra and XMM-Newton surveys are now providing strong additional constraints.

Optical identifications of the deepest Chandra and XMM-Newton fields are still in progress, however, a mixture of obscured and unobscured AGN with an increasing fraction of obscuration at lower flux levels 