  This chapter outlines the general principles for the detection and
characterisation of high-energy $\gamma$-ray photons in the energy range from
MeV to GeV. Applications of these fundamental photon-matter interaction
processes to the construction of instruments for $\gamma$-ray astronomy are
described, including a short review of past and present realisations of
telescopes. The constraints encountered in operating telescopes on
high-altitude balloon and satellite platforms are described in the context of
the strong instrumental background from cosmic rays as well as astrophysical
sources. The basic telescope concepts start from the general collimator
aperture in the MeV range over its improvements through coded-mask and Compton
telescopes, to pair production telescopes in the GeV range. Other apertures as
well as understanding the measurement principles of $\gamma$-ray astrophysics
from simulations to calibrations are also provided.
