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 few years thanks to the sizeable number of XMM-Newton and Chandra surveys, complemented by multiwavelength follow-up programs. I will review some of the recent results and the ongoing efforts aimed at obtaining a complete census of accreting Black Holes in the Universe and in particular: i) The multiwavelength searches for the most heavily obscured, Compton Thick, AGN; ii) The space density of high redshift (z ~ 3-5) X-ray selected AGN and the quest for early (z > 6) Black Holes The perspectives for future observations with present (XMM-Newton and Chandra) and future missions (i.e. SimbolX and IXO ) will also be outlined.