Active galaxies are among the most exciting and challenging phenomena in modern astrophysics. They are the most powerful and luminous objects in the Universe. They possess in their nucleus an amazing power source, pouring out the same energy as that emitted by a thousand Milky-Way-like galaxies (up to 1048 erg s−1). It is even more amazing that this source occupies a volume of space only the size of the solar system (i.e. ∼ 1015 cm). Many competing theories have endeavoured to explain this intriguing phenomenon. It is now commonly admitted that active galactic nuclei (AGNs) are powered by supermassive black holes and that accretion of gas into this central black hole is at the origin of their incredible luminosity (e.g. Rees, 1984). The accretion flow is thought to be the source of the X-ray, ultraviolet, and optical continuum emission. This emission ionizes the gas located in both the broad and narrowline regions and may also be the source of winds and jets.