Impulsivity is a complex trait that varies across healthy individuals, although when excessive, it is generally regarded as dysfunctional. Impulsive behavior may lead to initiation of drug addiction that interferes with inhibitory controls, which may in turn result in facilitation of the individual’s impulsive acts. Although environmental factors play a considerable role in impulsive behavior, a body of evidence collected in twin studies suggests that about 45% of the variance in impulsivity is accounted for by genetic factors. Genetic variants studied in association with impulsivity include those fortryptophan hydroxylase 1 and 2 (TPH1 and TPH2), the serotonintransporter (SERT), serotonin receptors, and genes of the monoamine metabolism pathway (e.g., monoamine oxidase A, MAOA). Other systems may also play a role in these behaviors, such as the dopaminergic system (the dopamine receptors DRD2, DRD3, and DRD4, and the dopamine transporter, DAT), the catecholaminergic system (catechol-O-methyltransferase, COMT), and the GABAergic system (GABAreceptor subunit alpha-1, GABRA1; GABA receptor subunit alpha-6, GABRA6; and GABA receptor subunit beta-1, GABRB1). Taking into account involvement of the hypothalamic-pituitary-adrenal (HPA) axis, the number of candidate genes implicated in impulsivity may be increased significantly and, therefore, may go far beyond those of serotonergic and dopaminergic systems. For a number of years, our group has conducted studies of the association of genes involved in the modulation of the stress-responsive HPA axis and several neurotransmitter systems, all involved in the pathophysiology of anxiety and depressive disorders, impulse control and compulsive disorders, with drug addiction. These genes include those of the opioid system: the mu- and kappa-opioid receptors (OPRM1 and OPRK1) and the nociceptin/orphaninFQ receptor (OPRL1); the serotonergic system: TPH1 and TPH2 and the serotonin receptor 1B (5THR1B); the catecholamine system: COMT; the HPA axis: themelanocortin receptor type 2 (MC2R or adrenocorticotropic hormone, ACTHR); and the cannabinoid system: the cannabinoid receptor type 1 (CNR1). In this chapter we will focus on these findings.