Spectral properties of X-ray background sources

In this thesis we analysed the deepest XMM-Newton observations of the Chandra Deep Field South and the Lockman Hole. These two reference fields for studies of different X-ray background source classes, provide a unique opportunity to study physical and evolutionary properties of distant normal and active galaxies, clusters of galaxies and even faint Galactic stars using statistically significant samples. The main goal of the work was a further improvement of the knowledge about X-ray properties of AGN and galaxies in the distant Universe using the statistical treatment of these large samples of detected sources.