  It is widely believed that most of the cosmic X-ray background (XRB) is
produced by a vast, hitherto undetected population of obscured AGN. Deep X-ray
surveys with Chandra and XMM will soon test this hypothesis. Similarly, recent
sub-mm surveys with SCUBA have revealed an analogous population of
exceptionally luminous, dust-enshrouded {\em star-forming} galaxies at high
redshift. There is now growing evidence for an intimate link between these
obscured populations. There are currently large uncertainties in the models,
but several independent arguments lead to the conclusion that a significant
fraction of the SCUBA sources ($10-30% $) will contain quasars. Recent
observational studies of SCUBA survey sources appear to confirm these
predictions, although the relative roles of AGN and star-forming activity in
heating the dust are unclear. Forthcoming surveys combining X-ray and sub-mm
observations will provide a very powerful tool for disentangling these
processes.
