There is accumulating evidence that impairment of neuropsychological and basic information processing abilities is an important feature of schizophrenia. Despite the size of this literature and persistent controversy on many specific points, there are several key reliable findings that are relevant to everyday clinical practice. First, impaired cognition is a fundamental aspect of the illness. It can be detected in nearly every case when appropriate controls are used and is often present from before the onset of major signs of the illness. Impairment of most functions is stable even in the face of clinical-symptomatic change. Thus, impairment is not an epiphenomenal feature of symptomatic state, although it is probably a good measure of the overall severity of the illness. In contrast to the perspective of many clinicians, the florid symptoms of schizophrenia are quite unlikely to be the cause of most cognitive deficits. Impairments of attention, memory, and problem solving appear to have particular neurobiologic significance. Although impairments of these functions are not diagnostic of schizophrenia, they are observed frequently and provide converging evidence that schizophrenia is fundamentally an illness of the brain and that cognitive impairment is a symptom domain in its own right, with as much importance for understanding and treating the disorder as positive or negative behavioral symptoms.