Extraterrestrial γ-rays tell us about the most energetic and violent phenomena in the Universe. Three features characterise them as unique messengers: i) copious production in both hadronic and electromagnetic interactions; ii) (almost) free propagation over the substantial fraction of the Universe; iii) effective detection by space-borne and ground-based instruments. The respective research area —Gamma Ray Astronomy— being a part of Astroparticle Physics, is a discipline in its own right. It addresses a broad range of high-energy processes of particle acceleration, propagation, and radiation on all astronomical scales: from compact neutron stars and black holes to large-scale cosmological structures. γ-ray studies cover a wide range of areas linked to the physics and astrophysics of SNRs, Star Formation, Magnetospheres of Pulsars and Black Holes, physics of relativistic outflows —Pulsar Winds, AGN jets, GRBs, etc. The major objectives of Gamma Ray Astronomy are connected, in one way or another, to the “centuries-old mystery” of Origin of Cosmic Rays. Therefore, identifying the main CR contributors with different astronomical source populations is considered one of the top priorities of high-energy astrophysics. The search for the sites of Cosmic Ray production (“Cosmic Ray Factories”) can be done best with space-borne and ground-based γ-ray detectors covering unprecedented energy range over ten energy decades, from MeV to PeV energies. Studies of the physics of CR factories, especially the so-called extreme accelerators responsible for the production of particles up to ≥ 10¹⁵ eV in our Galaxy (the so-called PeVatrons) and ≥ 10²⁰ eV in extragalactic objects (the so-called ZeVatrons) are of particular interest. In this lecture, I briefly describe the status of γ-ray studies, emphasising the recent exciting results obtained in the Very-High-Energy (VHE) and Ultra-High-Energy (UHE) bands in the context of Origin of Cosmic Rays.