The Demography of Super-Massive Black Holes: Growing Monsters at the
  Heart of Galaxies
  Supermassive black holes (BHs) appear to be ubiquitous at the center of all
galaxies which have been observed at high enough sensitivities and resolution
with the Hubble Space Telescope. Their masses are found to be tightly linked
with the masses and velocity dispersions of their host galaxies. On the other
hand, BHs are widely held to constitute the central engines of quasars and
active galactic nuclei (AGN) in general. It is however still unclear how BHs
have grown, and whether they have co-evolved with their hosts. In this Review I
discuss how, in ways independent of specific models, constraints on the growth
history of BHs and their host galaxies have been set by matching the statistics
of local BHs to the emissivity, number density, and clustering properties of
AGNs at different cosmological epochs. I also present some new results obtained
through a novel numerical code which evolves the BH mass function and
clustering adopting broad distributions of Eddington ratios. I finally review
BH evolution in a wider cosmological context, connecting BH growth to galaxy
evolution.
