Extragalactic X-ray surveys: AGN physics and evolution

We review the most important findings on AGN physics and cosmological evolution as obtained by extragalactic X-ray surveys and associated multiwavelength observations. We briefly discuss the perspectives for future enterprises and in particular the scientific case for an extremely deep (2-3 Ms) XMM survey. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.