Very high energy (VHE) -ray astronomy (defined here as observations at energies above 300 GeV and below 100 TeV) became viable with the development of the atmospheric Cerenkov imaging technique, which crossed the vital detection threshold with the detection of the Crab Nebula 10 years ago (Weekes et al. 1989). Although at the periphery of the observable electromagnetic spectrum, the VHE band must now be considered a legitimate astronomical discipline with established sources - both steady and variable, both Galactic and extragalactic - a growing number of observatories, and the promise of significant advances in detection techniques in the next few years. The confirmation of the detection of the Crab Nebula by more than eight groups over the past decade has given credibility to the existence of sources of TeV -rays and led to a rapid improvement in the sensitivity of ground-based -ray detection techniques.
The challenge therefore in this short review, which is aimed at the general astronomical community, is to convince the reader that there are viable methods of detecting -rays of energy 300 GeV and above from the ground, that a population of credible sources of various classes have been detected, and that these detections make a significant contribution to the astrophysics of high-energy sources. Although, as we shall see later, there are plans to extend the ground-based detection techniques down to energies of 20 GeV, and there are viable air shower experiments that operate at energies of 50 TeV and above, this review will focus on observations in the 300 GeV-30 TeV energy range; this has been the most studied energy band because it is easily accessible to the atmospheric Cerenkov imaging technique.