  Recent results have questioned the description of the QSO luminosity function
in terms of a pure luminosity evolution and call for a luminosity dependent
luminosity evolution. Measurements of the QSO clustering amplitude and
evolution allow further distinguishing among the various physical scenarios
proposed to interpret the QSO phenomenon. The general properties of the QSO
population would arise naturally if quasars are short-lived events connected to
a characteristic halo mass $\sim 5 \cdot 10^{12}$ M$_\odot$. This is the
typical mass of groups of galaxies in which the interactions triggering the QSO
activity preferentially take place.
