  The energy density of the Cosmic X-ray background (XRB) peaks around 30 keV
(see Figure 1), an energy not yet probed by focussing imaging instruments. The
first hard X-ray telescope due to fly on a space mission will be that on board
Constellation-X. The imaging capability, besides providing an improvement of
several orders of magnitude in sensitivity over current passively collimated
detectors, will permit for the first time to resolve a fraction of the XRB at
this most crucial energy. Synthesis models of the XRB based on obscured AGN
predict that at least 40% of the 10-40 keV XRB will be resolved by
Constellation-X.
