This book provides an up-to-date and comprehensive account of quasars and active galactic nuclei (AGN). The latest observations and theoretical models are combined in this clear, pedagogic textbook for advanced undergraduates and graduate students. Researchers will also find this wide-ranging and coherent review invaluable. Throughout, detailed derivations of important results are provided to ensure the book is self-contained. Theories and models are critically compared with detailed and often puzzling observations from across the spectrum. We are led through all the key topics, including quasar surveys, continuum radiation, time variability, relativistic beaming, accretion disks, jet sidedness, gravitational lensing, unification and detailed, multi-wavelength studies of individual objects. Particular emphasis is placed on radio, X- and gamma-ray observations - not covered in depth in any previous book. All those entering into this exciting and dynamic area of astronomy research will find this book an ideal introduction.