Aharonian (Max-Planck-Institut fur Kemphysik) notes that the time of saying a window is opening in the field of gamma ray domains in cosmic radiation is over; with maps of the gamma-ray sky and other rapidly-progressing research the window is wide open. Aharonian shows how this exploration of non-thermal phenomena has advanced in recent years, especially in theories of the origin of cosmic rays, the physics and astrophysics of relativistic jets, and the results of observational gamma-ray cosmology. He examines cosmic gamma rays in terms of production and absorption, their presence in supernova remnants, pulsars, pulsar winds and plerions, and their role in microquasars, as nonthermal phenomena in clusters of galaxies, TeV blazars and cosmic background radiation. He also examines high energy gamma rays as carriers of unique cosmological information. Appendices include background information in gamma-ray technology