A psychotic reaction marked by more or less systematized delusions without personality disorganization or deterioration. The term was first applied to persecutory and grandoise states by Karl Ludwig Kahlbaum in 1863, and became a separate clinical entity through the work of Emil Kraepelin. It derives from the Greek words “para” (beside, in the sense of change) and “nous” (intellect, reason), and was originally used to designate mental disorder in general.Two types of paranoid reactions are currently distinguished. In paranoia (“true paranoia”), the delusions develop insidiously and gradually crystallize into an intricate, rigid, and logical system. Hallucinations are usually absent, and delusions of persecution and/or grandeur dominate a relatively intact personality that appears normal in other respects. In paranoid states the delusions are transient, less systematic and logical, and are sometimes accompanied by hallucinations.Paranoid reactions comprise less than 1 per cent of all first admissions, but hospital statistics do not reflect their actual incidence, since many paranoiacs can exercise enough self-control to avoid commitment. They probably constitute a large proportion of the cranks, bigots, self-appointed saviors, fanatical reformers, and chronic litigants in our population. At present the male-female ratio is about equal, though these disorders were at one time more common among men. The average age at first admission is fifty, with most cases ranging from twenty-five to sixty-five. Their intellectual and educational level is usually above average.