In this book we discuss the problems encountered for calibrating space-borne instruments in the spectral range 50–300 nm. We also discuss the level of accuracy that can be achieved, and review the different sources of emissions that can be used in that range, i.e. the interplanetary background, stars, and solar system bodies such as planets, moons, and comets. As most of the work presented here is based on comparisons with older datasets obtained by experiments that have been calibrated independently, we also analyse the consistency between existing datasets. Whenever possible, we suggest a correction factor if an older dataset is in disagreement with a more recent one. For example, a comparison of the interplanetary background data acquired by the two ultraviolet (UV) spectrometers on Voyager 1 and Voyager 2 with models and other more recent datasets shows that it is indeed necessary to derive new calibration factors for these two instruments.