Book excerpt: Damage to the brain can impair language in many different ways, severely harming some linguistic functions whilst sparing others. To achieve some understanding of the apparently bewildering diversity of language disorders, it is necessary to interpret impaired linguistic performance by relating it to a model of normal linguistic performance. Originally published in 1987, this book describes the application of such models of normal language processing to the interpretation of a wide variety of linguistic disorders. It deals with both the production and the comprehension of language, with language at both the sentence and the single-word level, with written as well as with spoken language and with acquired as well as with developmental disorders.