The term active galactic nuclei refers to quasars, radio galaxies, Seyfert galaxies, blazars and related objects, all of which are believed to share a similar central engine - a huge black hole many times the mass of the Sun. This book aims to sum up leading ideas from across the range of research. It starts by addressing basic questions about the form, foundation and evolution of these structures, assessing the evidence for massive black holes and considering how they generate power by accretion. The author discusses X-ray and gamma-ray emission, radio emission and jets, emission and absorption lines, anisotropic appearance, and the relationship between an active nucleus and its host galaxy. The text explores the mysteries of what ignites, fuels and extinguishes active galactic nuclei, concluding with a general review of where the field stands now. The book pays careful attention to relevant physics as well as astronomy and should be of use to astronomers, physicists interested in applications of the theory of gravitation, and graduate students.