The study of X-ray emission from AGN allows the investigation of the inner regions of their central engine. By understanding the processes taking place within AGN, many other areas of research will benefit, such as accretion and the formation and evolution of galaxies.\ud This thesis begins by investigating the background AGN population of the serendipitous sources detected in a deep-look (~ 500 ks) of the XMM-Newton field of view surrounding 3C 273. The deep-look is created by mosaicing XMM-Newton EPIC calibration observations of 3C 273. Time averaged spectra of the 17 brightest X-ray sources are analysed. Cross-correlation of the X-ray sources with catalogues at other wavelengths is performed and unique candidate counterparts proposed. Classification of the X-ray sources is attempted and the background AGN population is discussed.\ud A number of AGN have shown evidence of high-ionisation, high-velocity outflows in their X-ray spectra. These sources are believed to have a high-accretion rate at or near the Eddington limit. Six suspected high-accretion rate narrow line AGN observed with XMM-Newton are analysed and inspected for the presence of these outflows. No such outflows are detected. However, three of the targets did show evidence of mass loss in the form of a warm absorber. In addition, all of the sources appear to have a soft excess below ~2 keV.\ud Finally, a search for the AGN population of super soft X-ray sources (SSXs) in the first and second XMM-Newton catalogues is performed. Hardness ratios are used to search for SSXs, which are defined as having a characteristic temperature, kT>~100 eV.\ud Cross-correlation of the selected SSXs with catalogues at other wavelengths is performed, returning both Galactic and extragalactic candidate counterparts. For a small number of SSXs with previously unclassified optical candidate counterparts, optical spectroscopic observations were acquired. The analysis of these observations identified 5 previously unclassified AGN
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