Scientific Colloquium
January 28, 2005

It has been known for over 30 years that quasars (also known as active galaxies or AGN) are radiating, massive black holes in the centers of galaxies. It has now become clear that the black holes and the galaxies are strongly connected and that black holes must have strongly influenced the formation of all structure in the universe. In order to understand this phenomenon we must first know how many AGN there are, how much luminosity they radiate and how these quantities evolve with cosmic time. The recent Chandra and XMM X-ray surveys have revolutionized our knowledge of AGN numbers and evolution. I will summarize recent progress in this area showing that we had, in the past,  seriously underestimated the number of AGN and misestimated their evolution.