Photostimulable x-ray storage phosphors in form of image plates are a promising alternative to conventional two-dimensional x-ray detectors. By absorption of ionizing radiation, electrons and holes are generated and captured locally to form a dose proportional latent image. The stored information is read out by scanning with a focussed HeNe laser. This leads to a local excitation of the trapped electrons which subsequently recombine with the trapped holes causing the emission of light. The information is then recorded by means of a photomultiplier, digitized by an A/D converter and displayed with the aid of a computer. The present state of the field of storage phosphors is reviewed in the following areas: the image principle, today's understanding of the nature of the storage centers, their physical generation, and the optically stimulated charge transfer paths. In addition, improvements and applications of these phosphors will be discussed for existing and future commercial devices.