What is AGN feedback ? AGN feedback is a mechanism generated by an active galactic nucleus to regulate the star formation activity in its host galaxy and the accretion onto the central black hole. Such a mechanism provides a solution to several cosmological problems regarding galaxy formation and evolution. In this talk, we will review these issues and the proposed solutions. We will then report the observational evidence that provides support and insights on such a mechanism. In particular, we will show recent results obtained by mapping the molecular and the ionized gas in massive galaxies at early epochs.