The phenomenon of photorefraction was discovered in 1966 in studies of propagation of a fairly powerful laser beam through the electro-optic crys- tals LiNb0 , LiTa0 , and some other compounds. The laser beam illumi- 3 3 nating part of the sample was found to cause a local change in the refrac- tive index of the crystal, thereby leading to distortion of the beam's wave front. The light had deteriorated the initially high optical quality of the crystal, in other words, it had given rise to a nonuniform distribution of the refractive index in the illuminated region. The effect was first called "opti- cal damage". The practical significance of the phenomenon was soon appreciated, applications were proposed, and a . vast amount of activity began. In the years that followed, the phenomenon was termed the "photorefractive ef- fect". Because of the reversible behavior of the refractive index variations due to photorefraction, photorefractive crystals have been regarded as re- cyclable photosensitive media. This became a valuable finding for optical engineers engaged in holography and optical information processing.On the other hand, the research into the nature of the photorefractive effect proved to be of considerable interest to physicists working in the fields of solid-state physics, semiconductors, and coherent optics.