This book is a fully updated new edition of the authors' earlier volume published in 1986, which covers both the background science and the current state of the art in the field of gamma ray astronomy. The emphasis is on the astronomy and astrophysics of known sources of cosmic gamma-rays outside the solar system. The authors discuss the mechanisms for the production and absorption of gamma-rays. The gamma-ray line astronomy of the interstellar medium, galactic center, and various discrete sources is then considered. Gamma-ray bursts are treated in considerable detail in chapter three, and the final two chapters describe medium energy and ultra-high energy gamma-rays. Relevant data from supernova SN1987A is also included in this edition. The concentration on phenomenology makes this book a fine introduction to gamma-ray astronomy.