  In-orbit background is an unavoidable feature of all space-borne X-ray
detectors, and arises both from cosmic sources (diffuse or point-like) and from
the interaction of the detectors themselves with the space environment (primary
or secondary cosmic rays, geomagnetically trapped particles, activation of
spacecraft structures). In this chapter the main background sources are
discussed, with their principal effects on the various detector types, and
simulation and mitigation strategies are described.
