Most cosmic accretion onto super-massive black holes is obscured by gas 
and dust that absorbs all but the hardest X-ray photons. By performing 
deep observations at very high energies, E>10 keV, it is possible to 
obtain a complete sample of the AGN population, including the elusive 
Compton thick sources. In this talk I present a study of the population 
of Compton Thick AGN and its relationship to the extragalactic X-ray
background. We found that in the local Universe, Compton Thick AGN are 
less numerous than previously expected and contribute only ~5-10% of 
the extragalactic X-ray radiation. Finally, I make a case for the 
existence for ultra-massive black holes in the Universe, but argue that 
there exists a likely upper limit to black hole masses of the order of 
~10^10 solar masses. Models of self-regulation that explain the 
co-evolution of the stellar component and nuclear black holes naturally 
provide such an upper limit.