Preface1 Introduction1.1 \"The Last Electromagnetic Window\"1.2 Energy Domains of Gamma Ray Astronomy1.3 Gamma Ray Astronomy�� A Discipline in Its Own Right 2 Status of the Field2.1 Low Energy Gamma Ray Sources2.1.1 The COMPTEL source catalog2.2 High Energy Gamma Ray Sources2.2.1 GeV blazars2.2.2 GeV pulsars2.2.3 Unidentified EGRET sources2.3 The Status of Ground-Based Gamma Ray Astronomy2.3.1 Brief historical review2.3.2 Reported TeV sources2.3.2.1 The Crab Nebula2.3.2.2 Other plerions2.3.2.3 Gamma ray pulsars2.3.2.4 Gamma rays from supernova remnants2.3.2.5 Other galactic sources2.3.2.6 TeV blazars2.3.2.7 Other extragalactic objects2.3.3 Next generation of I ACT arrays2.3.3.1 Atmospheric Cherenkov radiation2.3.3.2 Stereoscopic detection of Cherenkov images2.3.3.3 IACT arrays2.3.3.4 Sub-10 GeV ground based detectors? 2.3.3.5 Large field-of-view detectors2.3.3.6 IACT arrays for probing PeV y-rays3.Gamma Ray Production and Absorption Mechanisms3.1 Interactions with Matter3.1.1 Electron bremsstrahlung and pair-production3.1.2 Electron-positron annihilation3.1.3 Gamma rays produced by relativistic protons3.1.3.1 7rO-decay gamma rays3.1.3.2 Nuclear gamma ray line emission3.2 Interactions with Photon Fields3.2.1 Inverse Compton scattering3.2.2 Photon-photon pair production3.2.3 Interactions of hadrons with radiation fields3.3 Interactions with Magnetic Fields3.3.1 Synchrotron radiation and pair-production3.3.2 Synchrotron radiation of protons Relativistic Electron-Photon Cascades4.Gamma Rays and Origin of Galactic Cosmic Rays4.1 0rigin of Galactic Cosmic Rays�� General Remarks4.1.1 What do we know about Cosmic Rays? 4.1.2 What we do not know about Cosmic Rays? 4.1.3 Common beliefs and \"nasty\" problems4.1.4 Searching for sites of production of GCR,s4.2 Giant Molecular Clouds as Tracers of Cosmic Ray4.2.1 Proton fluxes in the ISM near the accelerator4.2.1.1 Impulsive source4.2.1.2 Continuous source4.2.1.3 The case of dense gas regions4.2.2 Gamma rays from a cloud near the accelerator4.2.3 Accelerator inside the cloud���� 5.Gamma Ray Visibility of Supernova Remnants6.PuLsars, Pulsar Winds, Plerions7.Gamma Rays Expected from Microquasars8.Large Scale Jets of Radio Galaxies and Quasars9.Nonthermal Phenomena in Clusters of Galaxies10.TeV Blazars and Cosmic Background Radiation11.High Energy Gamma Rays - Carriers of Unique Cosmological InformationAppendix A Spherically symmetric diffusion from a single sourceAppendix B Evolution of relativistic electrons in an expanding magnetised mediumBibliographyIndex