Gamma ray astronomy, the branch of high energy astrophysics that studies the sky in energetic y-ray photons, is destined to play a crucial role in the exploration of nonthermal phenomena in the Universe in their most extreme and violent forms. The great potential of this discipline offers impressive coverage of many "e;hot topics"e; of modern astrophysics and cosmology, such as the origin of galactic and extragalactic cosmic rays, particle acceleration and radiation processes under extreme astrophysical conditions, and the search for dark matter. The recent observational results and exciting theoretical predictions provide a strong rationale for a deep study of cosmic radiation with forthcoming satellite-borne and ground-based detectors in the so-called very high energy domain of the electromagnetic spectrum above 1010 eV.This invaluable book presents the motivations and highlights the principal objectives of the field, as well as demonstrates its intrinsic links to other branches of high energy astrophysics. Preference is given to three topical areas: (i) origin of cosmic rays; (ii) physics and astrophysics of relativistic jets; (iii) observational gamma ray cosmology. Also, an essential part of the book is devoted to the discussion of the principal mechanisms of production and absorption of energetic y-rays in different astrophysical environments, as well as to the description of the detection methods of high energy cosmic y-radiation.