Schizophrenia commonly has a chronic course albeit with fluctuating patterns, and cognitive disability. Its hallmark is psychosis, mainly characterized by positive symptoms such as hallucinations and delusions that are frequently accompanied by negative (deficit) symptoms such as reduced emotions, speech, and interest, and by disorganization symptoms such as disrupted syntax and behavior. Severe mood symptoms, up to and including manic and major depressive episodes, are present in many cases. There are no diagnostic laboratory tests for schizophrenia; instead, the diagnosis relies on clinical observation and self-report. It is then remarkable that ongoing epidemiological study over the last century using the clinical phenotype, but with variable ascertainment and assessment rules, has consistently shown the importance of genetic factors in schizophrenia.