This collection of surveys addresses graduate and predoctoral students. It reports on theoretical research and observational data on active galactic nuclei: The enigma of the nuclei of galaxies with their central "monster" driving the vast range of recent measurements in the infrared and radio region on our knowledge of the nucleus of our Galaxy; the spectra and classification of active galactic nuclei, the properties of thei host galaxies, their cosmological distribution and evolution, the role of stars and the hydrodynamics of the interstellar medium in the nuclei; the description of the inner parsec of a standard active galactic nucleus based on direct interpretation of the observations; the infrared activity of galaxies; the physics of radio galaxies and their jets, emphasizing the physics of gas flow and high-energy particle interactions as well as shock acceleration. These are all discussed in depth and presented in self-contained chapters with reference lists of the scientific literature.