  Gamma-ray telescopes in space are bombarded by large fluxes of charged
particles, photons and secondary neutrons. These particles and radiation pose a
threat to the nominal operation of satellites and limit the detection
sensitivity of gamma-ray instruments. The background noise generated in
gamma-ray space detectors by impinging particles is always much higher than the
astrophysical signal to be detected. In this chapter, we present the different
types of orbits suitable for gamma-ray missions, discussing their advantages
and disadvantages, as well as the value of experiments embarked in
stratospheric balloons. We then review the physical properties of all the
background components in the different orbits and the stratosphere.
