Abstract. Photorefraction is a nonlinear optical effect in which large nonlinearities can be observed at relatively low optical power levels. This phenomenon has evolved from a laboratory curiosity to a useful device technology for optical information processing applications. In particular, the abilities to form holograms in real time and also to store thern for extended periods of time have aided the implementation of several pattern-reccgnition machines that are adaptive and possess a high degree of parallelism. This paper reviews the photorefractive effect as a device technology for the problem of pattern recognition and describes a photorefractive realization of an adaptive pattern classifier.