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.  As a consequence, the search for, and the 
characterization of the evolutive and physical properties of (obscured) 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. After a brief review on how and why X-ray 
surveys are critical in studying and characterizing AGN, I will focus on the 
most recent results on AGN-galaxy co-evolution and in particular on observed 
AGN and galaxy properties. I will also present the science cases for 
observations at longer wavelengths and show how these new data may be useful to 
discriminate among different models of AGN triggering towards a better 
understanding of the co-eval AGN-galaxy growth.