Gamma-ray astronomy at high and very high energies

Over the last 10 years, important progress has been made in gamma-ray astronomy above 100 MeV, first with the EGRET detector on board the Compton Gamma-Ray Observatory satellite up to several tens of GeV, then with ground-based 'Atmospheric Cherenkov Telescopes' (ACTs), which have extended the spectral domain up to about 20 TeV. In this article, techniques implemented in these experiments are described; observations of high and very-high-energy gamma -ray sources are summarized together with their interpretations; finally, prospects opened by forthcoming projects in this new field of astrophysics are presented. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.