This chapter focuses on schizophrenia. Schizophrenia is a complex mental illness characterized by acute phases of delusions, hallucinations and thought disorder and chronically, by apathy, flat affect and social withdrawal. The IQ of schizophrenic patients is significantly lower than that of controls. Schizophrenic patients experience specific cognitive impairments. Findings demonstrating that intellectual impairment is more marked in some domains than others support the notion that circumscribed brain abnormalities may be associated with schizophrenia. Schizophrenic patients have difficulty in initiating and completing everyday tasks, being easily distracted and tending to give up when confronted by any obstacles. It is currently widely accepted that schizophrenia has a biological etiology. However, the shift toward this agreement is recent and the etiology of schizophrenia has been the subject of lengthy and intense debate. The diversity of symptoms in schizophrenia makes it unlikely that the pathophysiology can be accounted for by a single localized brain dysfunction. Instead, the strategy of attempting to localize specific symptoms to specific brain regions or connections between regions is encouraging. Schizophrenia is a heterogeneous illness, comprising a variety of different symptoms. Using groups of patients defined by diagnosis may explain the inconsistent and equivocal results of functional imaging studies.