Schizophrenia or affective psychosis may each be defined broadly or narrowly. When schizophrenia is broadly defined, affective psychosis and other functional psychoses are less frequently diagnosed. Conversely narrowing the definition of schizophrenia increases the pool and heterogeneity of affective psychoses, and increases interest in other functional psychoses. This chapter covers the historical development of the concept of psychosis, some historical concepts, common factors, current classification, DSM-5 diagnostic criteria, epidemiology, treatment, and culture bound syndromes. DSM-5 criteria made change the heading DSM-IV “schizophrenia and other psychotic disorder” to Schizophrenia spectrum and other psychotic disorder under which following disorder included are schizotypal (personality) disorder, delusional disorder, brief psychotic disorder, schizophreniform disorder, schizophrenia, and schizoaffective disorder. The treatment of other psychotic disorders is empirical and depends upon the conceptual orientation of the clinician. The commonly used medicines are antipsychotics, antidepressants, lithium, carbamazepine and sodium valproate in different combinations, doses and length of time.