  We present synthesis models of the X-ray background where the available X-ray
observational constraints are used to derive information on the AGN population
properties. We show the need for luminous X-ray absorbed AGNs, the QSO2s, in
reproducing the 2-10 keV source counts at relatively bright fluxes. We compare
a model where the evolution of absorbed AGNs is faster than that of unabsorbed
ones, with a standard model where absorbed and unabsorbed AGNs evolve at the
same rate. It is found that an increase by a factor of ~2 from z=0 to z~1.3 in
the ratio between absorbed and unabsorbed AGNs would provide a significant
improvement in the data description. Finally, we make predictions on the AGNs
to be observed in deep X-ray surveys which contain information on the AGN space
density at high redshift.
