Schizophrenia is a psychotic disorder that primarily affects thought and behaviour. Abnormal perceptions such as hallucinations are characteristic, as are abnormal beliefs such as delusions. The disorder is often chronic and relapsing. Usually, it can be well controlled with antipsychotic drugs sometimes called neuroleptics. Schizophrenia mainly affects thought and behaviour, as opposed to affect. It is often a chronic disorder in that there are multiple acute episodes and between these residual effects. The diagnosis of schizophrenia is usually made with the help of a longitudinal view of the patient that is the form of the illness is as important as the content of the illness in making a diagnosis. The chapter discusses the differential diagnosis of schizophrenia. Concepts about schizophrenia have changed over time. This may be because the disease itself changes with time, or because we are actually witnessing a phenomenon with several different causes presenting a common picture.