Gamma-ray astronomy has experienced a period of very impressive scientific advances and successes during the last decade. In the high-energy range studied with space instruments above 100 MeV, the AGILE and Fermi missions led to important discoveries. In particular, they have established an inventory of active sources of various kinds (blazars, pulsars, supernova remnants, high-mass binaries, gamma-ray bursts, etc.) showing a variety of gamma-ray emission processes. Important new development concerns the role of gamma-ray astronomy in the multi-messenger studies of the energetic astrophysical objects as mergers of relativistic objects – neutron stars and black holes detected as gravitational wave sources by LIGO and VIRGO observatories. Gamma-ray observations are crucial for studies of the origin of dark matter particles and sources of astrophysical neutrinos detected by the Ice Cube observatory. The operating Cherenkov observatories H.E.S.S., MAGIC, VERITAS, HAWC, LHAASO and the future CTA telescope array extend the gamma-ray observations to very high photon energies.

At the same time many of the most spectacular objects in the Universe have their peak emissivity at photon energies between 0.2 and 100 MeV (e.g., gamma-ray burst sources, blazars, pulsars, etc.), so it is in this energy band that essential physical properties of these objects can be studied most directly. This energy range is also known to feature spectral characteristics associated to gamma-ray emission from pion decay, the primary signature of hadronic acceleration. This fact makes the MeV energy region of paramount importance for the study of radiating nonthermal particles and for distinguishing leptonic processes from hadronic ones. Moreover, this energy domain covers the crucial range of nuclear gamma-ray lines produced by radioactive decays, nuclear collisions, positron annihilations, or neutron captures, which makes it as special for high-energy astronomy as optical spectroscopy is for manifold phenomena of atomic physics. In particular, measuring the redshifts of MeV regime nuclear lines in the close vicinity of relativistic objects – neutron stars and black holes – provides unique information on the inner radii of accreting regions and allows to constrain the equation of state of neutron stars. The highest magnetic fields, up to 10(15) G, well exceeding the critical characteristic fields of quantum electrodynamics, were revealed in magnetars. Thus their cyclotron lines have to be in the MeV range. The peak of synchrotron emission of gamma-ray binaries is expected to be in the MeV energy range. This opens the possibility to use the unique power of MeV spectroscopy to study fundamental physical processes.

However, due to the limited sensitivity of the past and present detectors, at the intermediate gamma-ray energies between 0.2 and 100 MeV only a few tens of steady sources have been detected so far, mostly by the COMPTEL instrument aboard the Compton Gamma-Ray Observatory and by the INTEGRAL observatory. Therefore, this field has remained largely unexplored. The ISSI workshop is devoted to a comprehensive analysis of all the known classes of astrophysical sources of MeV emission and their multi-wavelength properties with the aim to establish the important role of such objects for the stellar nucleosynthesis and chemical evolution of galaxies, for the physics of degenerate stars, for dark matter studies, and fundamental physics.