  The fields of cosmic ray astrophysics, gamma-ray astrophysics, and neutrino
astrophysics have diverged somewhat. But for the air showers in the GeV and TeV
energy ranges, the ground-based detector techniques have considerable overlaps.
VHE gamma-ray astronomy is the observational study measuring the directions,
flux, energy spectra, and time variability of the sources of these gamma rays.
With the low flux of gamma rays, and the background of charged particle cosmic
rays, the distinguishing characteristic of gamma-ray air shower detectors is
large size and significant photon to charge particle discrimination. Air shower
telescopes for gamma-ray astronomy consist of an array of detectors capable of
measuring the passage of particles through the array elements. To maximize
signal at energies of a TeV or so, the array needs to be built at high altitude
as the maximum number of shower particles is high in the atmosphere. These
detectors have included sparse arrays of shower counters, dense arrays of
scintillators or resistive plate counters (RPC), buried muon detectors in
concert with surface detectors, or many-interaction-deep Water Cherenkov
Detectors (WCD). In general these detectors are sensitive over a large field of
view, the whole of the sky is a typical sensitivity and perhaps 2/3 of the sky
selected for clean analysis, but with only moderate resolution in energy,
typically due to shower-to-shower fluctuations and the intrinsic sampling of
the detector. These telescopes though, operate continuously, despite weather,
moonlight, day or night, and without needing to be pointed to a specific target
for essentially a 100\% duty cycle. In this chapter we will examine the
performance and characteristics of such detectors. These are contrasted with
the Imaging Air Cherenkov Telescopes which also operate in this energy range,
and both current and future proposed experiments are described.
