Several observational pieces of evidence and theoretical arguments suggest that feedback from accreting black holes is an unavoidable ingredient in the formation of galaxies. Many of these arguments are very well addressed in this volume; here I only mention the recent result of a dearth of low-luminosity AGN in the GOODS deep fields [1]. Taken together with other datasets, like the recent COMBO17 [3] and SDSS [4] surveys, this result allows to reconstruct the AGN luminosity function at z~4 from M1450=-21 to -28. This is roughly fit by a simple (negative) density evolution of the z=3 luminosity function [2]. This fact highlights the apparently "anti-hierarchical" behaviour of AGN (see also A. Merloni, these proceedings), in the sense that the peak of bright quasars aniticipates that of fainter objects, at variance with what is expected if AGN simply followed the assembly of dark matter halos.
