The Distribution of X-Ray Emitting Quasars in Space /

This review is mainly concerned with the discussion of the statistical properties of X-ray and optically selected samples of quasars and Sy 1 nuclei and their intercomparison. The inconsistencies which have been found are clearly exemplified by the discrepancy between the observed and the predicted X-ray source counts. It is shown that a satisfactory understanding of these problems has not yet been reached. The last section deals with the long debated contribution of AGNs to the extragalactic X-ray background (XRB), which is still uncertain within a factor of 2, although a lower bound of 30% at the (now convenient) energy of 2 keV can be set.