The study of the physical processes at work in Active Galactic Nuclei (AGN) has kept a large number of astronomers busy since the discovery, in the early sixties, of the ?rst radio galaxies and quasars. After a few decades of debate there is now a clear consensus about the source of energy in AGN, namely gravitational energy released through matter accretion onto a massive black hole. The most commonly accepted ideas regarding AGN are pictured in...