The chapter addresses the molecular genetic basis of the personality trait positive emotionality (PE). Beginning with historical aspects of heritability estimation and personality assessment, the main portion of this chapter discusses the molecular genetics basis of PE, which is investigated far less frequently than that of negative emotionality (NE). The studies reviewed focus on individual differences in PE in healthy subjects and include only those studies that assess PE by broadly accepted personality inventories. The review is concentrated on dopaminergic and serotonergic genes because these genes show the most association with PE.