The science of the WG 4    The path leading from the formulation of Einstein field equations (1915) to the acknowledgment of the existence of black holes by the scientific community (in the sixties) is quite long. In between, at least two essential steps deserve to be mentioned ___ Schwarzschild's calculation of the curvature of spacetime caused by the gravitational field of a star (1916) and Chandrasekhar's demonstration of the existence of a maximum mass for stars that have exhausted their nuclear fuel (1930); above this limit, no force can oppose the gravitational attraction and the star must implode. In a very simplified way, we could say that, in the early thirties, most of the theoretical bases for claiming the existence of black holes were already set.    It took about 30 years before the scientific community accepted the idea that black holes could be something more than a theoretical artifice. Among the reasons that supported the general skepticism, there was the obvious dificulty in collecting evidence of their existence. How could astronomers find any sign of an object whose distinctive character is to entrap the light? It is almost ironic, then, to think that optical telescopes were pointing towards black holes already in the end of XIX century, long before the formulation of Einstein field equations. Astronomers were regularly observing black holes, but failed to recognize them, because in the optical band they looked just like ordinary stars. Now, these objects are known as quasars, a contraction of `quasi-stellar objects'.    As astronomical observing techniques became more sophisticated, the exceptional properties of quasars started to reveal themselves. The development of spectroscopy, along with the construction of the first radio and X-ray telescopes, played a key role in revolutionizing the understanding of quasars. The analysis of spectral lines unveiled that quasars are among the most distant objects in our universe. Given their distance and apparent luminosity, it was possible to deduce the amount of radiation they emit ___ of the order of hundreds of times the radiation produced by our whole Galaxy. The size of the emitting core resulted to be generally smaller than a light-year, which means at least 10000 times smaller than the Milky Way. Observations in the radio band revealed the physical process that dominates the quasar emission from radio to optical wavelengths ___ synchrotron radiation, produced by electrically charged particles moving at relativistic velocity (i.e. close to the speed of light) in presence of a magnetic field. The clues that astronomers collected about the nature of these extremely compact and luminous sources were many and sometimes very dificult to decipher, but it is now generally accepted that the engine that powers quasars ___ as well as the more general class of Active Galactic Nuclei (AGNs) ___ is a supermassive black hole, with mass of the order of 109M☉.    Our general knowledge about how AGNs form, how they evolve and die is still vague. To find an answer to all the open questions concerning them is not simply a matter of intellectual curiosity. Given the amount of energy produced in their core, AGNs can be regarded as an open window on the most extreme physical processes taking place in our universe.