In the last 15 years gamma-ray astronomy has undergone a tremendous development, changing from a rather marginal and exotic field to a key discipline that probes the most extreme manifestations of the physical universe. Through successful observational experiments like the Compton Gamma-Ray Observatory and modern Imaging Cherenkov Telescopes, several types of cosmic gamma-ray sources have been detected and are currently being subjected to intensive study. These sources include some of the most extraordinary objects known by human beings. Objects like accreting black holes, neutron stars and their winds, supernova remnants, distant gamma-ray bursts, and the central regions of quasars and blazars. This book presents the most updated review of our current knowledge of cosmic gamma-ray sources. Besides chapters devoted to each type of known gamma-ray emitting object, it provides a self-contained introduction to gamma-ray astrophysics, including a description of the relevant physical processes, a guide through the basic phenomenology, and the prospects for further advances. The book will be of interest for all active researchers in the field, for scientists interested in understanding the recent progress in gamma-ray astronomy, and for graduate students.