Between 3 and 8% of preschool children in Britain and the United States are delayed in their language development by more than a year below the normal range. Evidence suggests that such children are particularly impaired in acquiring syntactic and morphological rules, although well-defined syndromes of impaired language development have not yet been determined. The effects of preschool impairments in language development on subsequent school achievement are significant: 60% of these children are in special classes for the learning impaired at 9 years of age. Auditory processing deficits have been demonstrated in language-impaired children, but the contributions of such deficits to language development difficulties have yet to be determined. While autistic children have severe cognitive disorders and visual and auditory perceptual disturbances, the difficulties of nonautistic language-impaired children seem confined more to language expression and comprehension. Recent neuropsychological evidence concerning brain organization for language behavior suggests that these children have deficiencies in language functions which normally depend on left hemisphere functioning.