  We report the results of a detailed analysis of the contribution of various
classes of AGNs to the extragalactic X-ray background (XRB). The model is based
on the unification schemes of AGNs and on their related X-ray spectral
properties in the light of recent observational results. The integrated
emission from AGNs, when folded with an appropriate cosmological evolution law,
can provide a good fit to the XRB over a wide energy range, from several to
$\sim$ 100 keV, while it contributes only about 74\% of the ROSAT soft XRB. The
model predictions have been checked against all available observational
constraints from both hard and soft X-ray surveys (counts, redshift
distributions and average X-ray source spectral properties).
