  The ROSAT Deep Survey in the Lockman Hole is the most sensitive X-ray survey
performed to date, encompassing an exposure time of 200 ksec with the PSPC and
1.2 Msec with the HRI. The source counts reach a density of ~1000/deg2 at a
limiting flux of ~1E-15 erg/cm2/s. At this level 70-80% of the 0.5-2 keV X-ray
background is resolved into discrete sources. Because of the excellent HRI
positions, 83 X-ray sources with fluxes (0.5-2 keV) above 1.2E-15 erg/cm2/s
could be optically identified so far utilizing deep optical CCD images, NIR
photometry and Keck spectroscopy. Only 11 sources above this flux limit remain
unidentified. The majority of objects turned out to be active galactic nuclei
(AGN) with minority contributions of clusters of galaxies, stars and some
individual galaxies. These deep pencil beam data together with a number of
shallower ROSAT surveys define the source counts over six orders of magnitude
in flux and provide a unique tool of unprecedented quality to study the
cosmological evolution of AGN, which can easily provide the bulk of the
extragalactic X-ray background and could give an important contribution to the
total background light in the universe.
