  High energy gamma-ray astronomy has been established during the last decade
through the launch of the Compton Gamma Ray Observatory (CGRO) and the success
of its ground-based counterpart, the imaging atmospheric Cherenkov technique.
In the aftermath of their important and surprising scientific results a
worldwide effort developing and designing new generation atmospheric Cherenkov
detectors is underway. These novel instruments will have higher sensitivity at
E > 250 GeV, but most importantly, will be able to close the unexplored energy
gap between 20 GeV and 250 GeV. Several ground-based detectors are proposed or
under construction. Aspects of the techniques used and sensitivity are
discussed in this overview paper. The instruments cover largely complementary
energy ranges and together are expected to explore the gamma-ray sky between 20
GeV and 100 TeV with unprecedented sensitivity.
