  Indirect detection of gamma rays with ground-based observatories is currently
the most sensitive experimental approach to characterize the gamma-ray sky at
energies $>0.1$\,TeV. Ground-based detection of gamma-rays relies on the
electromagnetic showers that gamma rays initiate in the Earth's atmosphere. In
this chapter we will review the properties of electromagnetic air showers as
well as the differences with respect to cosmic-ray showers that enable the
rejection of the cosmic ray background. The experimental techniques that have
been developed for ground-based detection of gamma rays will be introduced.
These fall onto three main categories: air shower particle detectors, sampling
Cherenkov arrays, and imaging atmospheric Cherenkov telescopes. Hybrid concepts
as well as other experimental approaches are also discussed.
