The gas scintillation proportional counter has seen rapid development in recent years as a broad-band spectrometer for X-ray astronomy. It may be used in the conjunction with grazing incidence optics or coded aperture masks to produce spectral images of the X-ray sky. Its performance characteristics such as spatial resolution, energy resolution and background rejection efficiency will be described. Reference will be made in large part to work conducted over the last few years at SSD/ESTEC and to work undertaken by the Columbia and Los Alamos groups and at MSSL. The gas scintillator may be coupled to a variety of readout systems including an array of photomultipliers, a microchannel plate or a photoionisation detector. Their relative merits will be reviewed. At high pressures (∼ 3–5 atm) the imaging gas scintillator can be used in nuclear medicine, where its spatial resolution and energy resolution can provide higher resolution images with better contrast than the standard gamma camera.