  The hunt for cosmic TeV particle accelerators is prospering through Imaging
Atmospheric Cerenkov Telescopes. We face challenges such as low light levels
and MHz trigger rates, and the need to distinguish between particle air showers
stemming from primary gamma rays and those due to the hadronic cosmic ray
background. Our test beam is provided by the Crab Nebula, a steady accelerator
of particles to energies beyond 20 TeV. Highly variable gamma-ray emission,
coincident with flares at longer wavelengths, is revealing the particle
acceleration mechanisms at work in the relativistic jets of Active Galaxies.
These 200 GeV to 20 TeV photons propagating over cosmological distances allow
us to place a limit on the infra-red background linked to galaxy formation and,
some speculate, to the decay of massive relic neutrinos. Gamma rays produced in
neutralino annihilation or the evaporation of primordial black holes may also
be detectable. These phenomena and a zoo of astrophysical objects will be the
targets of the next generation multi-national telescope facilities.
