The nature of dyslexia is defined and reviewed in the context of neurological and educational processes and the nature of a written language system. An illustrative case history of a typical dyslexic child is presented. The nature of a script system is examined in the light of current linguistic and psycholinguistic studies. Noted are characteristics of the rule governed written language system such as scriptphonetic relationships. Stressed is the interface between the language system and maturational factors including unidirectional sensory and motor modes of perceiving. The incidence of dyslexia is related to the probabilities of individual differences in lateralization of brain function. It is concluded that dyslexia can be regarded as a primary difficulty based on the incompatibility between the written language system itself and the intrinsic developmental skills of an individual's perceptual motor system. (DB)