The Missing X-ray Background
  The fraction of the hard X-ray background (XRB) resolved into individual sources by the deep Chandra and XMM-Newton surveys strongly depends on the adopted energy range and decreases with increasing energy. As a consequence, the nature of the sources of the even harder (>10 keV) XRB remains observationally poorly constrained. I will briefly discuss the need for X-ray observations above 10 keV. 
A. Comastri (INAF-Osservatorio Astronomico di Bologna, Italy)