  The response of the X and Gamma Imaging Spectrometer (XGIS) instrument
onboard the Transient High Energy Sky and Early Universe Surveyor (THESEUS)
mission, selected by ESA for an assessment phase in the framework of the Cosmic
Vision M5 launch opportunity, has been extensively modeled with a Monte Carlo
Geant-4 based software. In this paper, the expected sources of background in
the Low Earth Orbit foreseen for THESEUS are described (e.g. diffuse photon
backgrounds, cosmic-ray populations, Earth albedo emission) and the simulated
on-board background environment and its effects on the instrumental performance
is shown.
