Active Galactic Nuclei (AGN) are compact regions at the centre of active galaxies which are visible across the full electromagnetic spectrum. Their radiation comes from the accretion of matter onto a supermassive black hole. They are particularly bright in X-rays, showing several components in their spectra. The origin of some components is still very debated, for example the nature of the excess of emission often detected in unobscured AGN below 2keV (“soft-excess”), or the location where reflection features are produced. A unified model, describing the global anatomy of an AGN, is commonly used to explain the different classes of these objects. Nevertheless, the structure of their inner regions is still uncertain, as well as the nature of their outflows. My PhD dissertation is dedicated to the study of AGN emission mechanisms and structure, in particular through multiwavelength analyses of the archetypal Seyfert 1 galaxies Mrk 509 and NGC 5548, and through statistical studies.