High Energy Large Area Surveys: optically obscured AGN and the history   of accretion
  Hard X-ray, large area surveys are a fundamental complement of ultra-deep, pencil beam surveys in obtaining a more complete coverage of the L-z plane, allowing to find luminous QSO in wide z ranges. Furthermore, results from these surveys can be used to make reliable predictions about the luminosity (and hence the redshift) of the sources in the deep surveys which have optical counterparts too faint to be observed with the present generation of optical telescopes. This allows us to obtain accurate luminosity functions on wide luminosity and redshift intervals. 
F. Fiore and the HELLAS2XMM collaboration