In the last decade, a combination of high sensitivity and spatial
resolution observations in the extragalactic domain, and of
coordinated multi-wavelength campaigns on galactic objects have
revolutionized our understanding of black holes astrophysics on all
mass scales. This has led to substantial progress towards
super-unification schemes for active galactic nuclei, in which the
large variety of different AGN classes can be classified based on
fundamental physical parameters only, such as mass, accretion rate and
kinetic energy output.
I will describe the constraints available from a study of AGN
evolution synthesis models on the growth of the SMBH population in the
two main 'modes' observed (`kinetic-' and `radiatively-dominated',
respectively).  SMBH always show a very broad accretion rate
distribution, and I'll discuss the consequences of this fact for our
understanding of observed AGN fractions in galaxies.
I'll show how SMBH mass function evolves anti-hierarchically, i.e. the
most massive holes grew earlier and faster than less massive ones, and
I will also derive tight constraints on the average radiative
efficiency of AGN.
Finally, constraints on the redshift evolution of the AGN kinetic
luminosity function will be briefly discussed and compared with the
radiative output of the evolving SMBH population, thus providing a
robust physical framework for phenomenological models of AGN feedback
within structure formation.