Over the last few years, the existence of mutual feedback effects between accreting supermassive black holes powering AGN and star formation in their host galaxies has become evident. This means that the formation and the evolution of AGN and galaxies should be considered as one and the same problem. As a consequence, the search for, and the characterization of the evolutive and physical properties of AGN over a large redshift interval is a key topic of present research in the field of observational cosmology. Significant advances have been obtained in the last ten years thanks to the sizable number of XMM-Newton and Chandra surveys, complemented by multi-wavelength follow-up programs. I will review how and why X-ray surveys are critical in studying and characterizing AGN, from the "cosmological" horizon down to the "event" horizon. I will focus on the most recent results (past 2 years) on AGN-galaxy co-evolution and in particular on observed AGN and galaxy properties that have been used in the recent years to constrain models of AGN triggering and evolution. I will also discuss the expected contribution of multi-wavelength follow-up observations towards a better understanding of the co-eval AGN-galaxy growth.