Cognitive impairments are present in most patients with schizophrenia and are a major determinant of functional impairments. Before trying to treat cognition with a goal of improving functional disability, it is important to determine that these deficits are not an artifact of other aspects of the illness. This chapter reviews the evidence regarding the primary nature of these cognitive impairments. After reviewing the literature, the conclusion that cognitive deficits are central feature of the illness appears supported. There is remarkably little evidence to suggest that these impairments are caused by other aspects of the illness, despite the flagrant nature of many of these symptoms.