  A review of recent developments in the field of X-ray surveys, especially in
the hard (2-10 and 5-10 keV) bands, is given. A new detailed comparison between
the measurements in the hard band and extrapolations from ROSAT counts, that
takes into proper account the observed distribution of spectral slopes, is
presented. Direct comparisons between deep ROSAT and BeppoSAX images show that
most hard X-ray sources are also detected at soft X-ray energies. This may
indicate that heavily cutoff sources, that should not be detectable in the
ROSAT band but are expected in large numbers from unified AGN schemes, are in
fact detected because of the emerging of either non-nuclear components, or of
reflected, or partially transmitted nuclear X-rays. These soft components may
complicate the estimation of the soft X-ray luminosity function and
cosmological evolution of AGN.
