Active galaxies are amongst the most exciting and challenging phenomena in modern astrophysics.
They are the most powerful objects we observe in the universe, encompassing a wide range of extreme physical conditions. This book reviews current research in this rapidly advancing field. In Active Galactic Nuclei, the author discusses our place in the Universe, basic ideas about galaxies and cosmology, and the main classes of active galaxy. He reviews observational techniques, their development and limitations, and emphasises the crucial link between observation and theory. Emission processes occurring in active galaxies are described and models of active galactic nuclei, including the black hole power source, are explored.