Infant temperament is theorized to have a strong genetic basis. Yet, studies of the stability of temperament and molecular genetics research on temperament have revealed inconsistent findings. One reason may be because research has not taken into account the influence of early social experiences. We review research on aspects of infant emotionality presumed to be temperamental in origin, emotion reactivity and emotion regulation, and integrate research addressing parenting influences on physiology and genetics of infant emotionality with traditional research on behavioral expression of emotionality. Empirical research supports the theory that sensitive and appropriately responsive parenting in infancy is related to more optimal patterns of behavioral and physiological reactivity and regulation. Molecular genetics research with both animal and human populations has produced consistent results when environmental experiences are taken into account. Findings suggest that the short allele of the serotonin transporter gene and the long allele of the DRD4 dopamine receptor gene may be related to distinct patterns of infant emotionality and to later problems with depression, impulse control problems, and externalizing/antisocial behaviors, primarily when paired with insensitive parenting, abuse or neglect, or other adverse family environments. Future directions and limitations in the field are discussed.