  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.
