Andrea Comastri (INAF-Osservatorio Astronomico di Bologna) on behalf of the HELLAS2XMM Team
Thanks to sensitive hard X-ray observations carried out with Chandra and XMM--Newton the census of the AGN population responsible of the X-ray background (XRB) has been almost completed. It is by now possible to investigate the cosmic evolution and the physical properties of the obscured AGN responsible of the energetically dominant component of the XRB. Despite extensive multiwavelength campaigns of large samples of X-ray selected AGN several issues (and in particular those concerning the redshift and absorption distribution) are still actively debated. I will review the most important achievements obtained combining deep and shallow Chandra and XMM surveys. In particular I will focus the discussion on the properties of X--ray luminous, obscured type 2 quasars Extensive follow--up observations in the near--infrared reveal a complex morphology of their host galaxies and open the possibility to probe the co--evolution of supermassive black holes and stellar component at relatively high redshifts. The overall properties of these sources need to be taken into account to obtain a robust estimate of the evolution of the AGN luminosity function.