  Hard X-ray observations are the most efficient way to discriminate
accretion-powered sources from star-light. Furthermore, hard X-rays are less
affected than other bands by obscuration. For these reasons the advent of
imaging instruments above 2 keV, has permitted to dramatically improve our
understanding of accretion-powered sources and their cosmic evolution. By
minimizing the problems of AGN selection and nuclear obscuration, the
combination of deep and shallow hard X-ray surveys, performed first with ASCA
and BeppoSAX and then with Chandra and XMM (e.g. ASCA LSS, HELLAS, CDFN, CDFS,
Lockman Hole, SSA13, HELLAS2XMM etc.), allows a detailed study of the evolution
of accreting sources. Somewhat surprising results are emerging: 1) the sources
making the Cosmic X-ray Background peak at a redshift (z=0.7-1) lower than soft
X-ray selected sources and lower than predicted by synthesis models for the
CXB; 2) there is strong evidence of a luminosity dependence of the evolution,
low luminosity sources (i.e. Seyfert galaxies) peaking at a significantly later
cosmic time than high luminosity sources.
