Physics and astrophysics with a ground-based gamma-ray telescope of low energy threshold

Ground-based Cherenkov telescopes have made in recent years important contributions to high energy gamma-ray astronomy. A lower energy threshold, considerably below 100 GeV, and improved sensitivity will be key parameters to extend their role. A lower threshold will permit these instruments to cover wavelengths with good overlap with satellite experiments, thus providing essential complementary information. The latest generation of Imaging Air Cherenkov Telescopes was built with this criterion in mind. Preliminary studies concerning further progress in the same direction have started. We discuss in this contribution the astrophysics and physics arguments for lowering the observable energy threshold as far as the Cherenkov technique permits, and the ensuing complementarity to results obtained with a GLAST-like satellite.