  The physical and evolutionary relation between growing supermassive black
holes (AGN) and host galaxies is currently the subject of intense research
activity. Nevertheless, a deep theoretical understanding of such a relation is
hampered by the unique multi-scale nature of the combined AGN-galaxy system,
which defies any purely numerical, or semi-analytic approach. Various physical
process active on different scales have signatures in different parts of the
electromagnetic spectrum; thus, observations at different wavelengths and
theoretical ideas all should contribute towards a "large dynamic range" view of
the AGN phenomenon. As an example, I will focus in this review on two major
recent observational results on the cosmic evolution of supermassive black
holes, focusing on the novel contribution given to the field by the COSMOS
survey. First of all, I will discuss the evidence for the so-called
"downsizing" in the AGN population as derived from large X-ray surveys. I will
then present new constraints on the evolution of the black hole-galaxy scaling
relation at 1<z<2 derived by exploiting the full multi-wavelength coverage of
the survey on a complete sample of ~90 type 1 AGN.
