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 the so-called AGN uni?ed model, although one should remember that this is still a “working” model. Tremendous progress has been made in unveiling, analyzing, and modeling the di?erent components in AGN: the accretion disk, the jets of relativistic particles, the x-ray absorber close to the central engine,the“torus”thatfunnelstheionizingradiation,thesurroundingclouds of dense material in the broadline region and narrowline region, the cool molecular and dusty material around the torus, as well as jet-induced e?ects on larger scales. Yet, it remains to incorporate all these components and physical processes into a self-consistent AGN model that would take into account, and predict, all the facets of AGN activity. The mere process of matter fueling is far from being totally understood and neither is the link of the AGN with its host galaxy. The birth and growth of massive black holes in the universe is a related question to be investigated and is of prime interest for cosmology.
