Image plates based on photostimulable X-ray storage phosphors establish a promissing alternative to conventionally utilized two-dimensional X-ray detectors. These image plates consist of a layer of powdered X-ray storage phosphor, usually the alkaline-earth halide BaFBr:Eu 2+, mixed with an organic binder and mounted on a polymeric film. Upon absorption of ionizing radiation, electrons and holes are produced efficiently. Both species are trapped in the immediate surrounding of their creation thereby forming a latent image. The readout of this image occurs by photostimulation whereby the trapped electrons are liberated and recombine radiatively with the trapped holes under emission of Eu 2+ light. In the present paper the principle of the imaging system is introduced followed by a brief review of the underlying physics explaining the nature of the involved traps and the mechanism of photostimulation. Advantages and disadvantages of image plate detectors and their physical nature are discussed and possible improvements suggested. Present and future fields of application are presented.