X-ray sources and the diffuse background

Possible numbers of X-ray sources at various flux levels are considered and their contribution to the background evaluated. Our conclusions are: The mean spectrum of Quasi-Stellar Objects and the nuclei of Seyfert galaxies between optical and X-ray wavelengths is characterized by a spectral index well in excess of 1.0. These objects may make an important contribution to the soft X-ray background. Observations of Q.S.O. may allow a decisive test for the cosmological interpretation of the redshifts of the Q.S.O. With regard to galactic sources we find that few undetected sources of the Sgr-Sco type exist, that these sources are located in the immediate neighborhood of the galactic center and that they account for at least 80% of the total galactic source luminosity. However, many intrinsically much fainter galactic sources may exist.