The Thales project 383549 "The dark side of the accretion history of the Universe" was jointly funded by the European Union and the Greek Government during the period 2013-2015 within the framework of the programme ``Education and lifelong learning''. A copy of the science proposal submitted to Greek Ministry of Education can be found here. The goal of the project is provide robust constraints on the whereabouts and evolution of Active Galactic Nuclei (AGN), including heavily obscured ones, by combining state-of-the-art statistical analysis techniques with the latest observations at different wavelengths.