The term insight lacks a universal definition, but in clinical practice with patients with psychotic disorders, it refers to a mental state inferred from the verbal and behavioral response of a patient to his or her illness. This article reviews the historical development of the concept, some explanatory theoretical models, and the clinical variables which have been related to insight, such as: severity of clinical manifestations, age of onset, prognostic, adherence to treatment, level of intelligence, and neuropsychological alterations.