Attention deficit hyperactivity disorder (ADHD) is a neuropsychiatric disorder whose three main symptoms are impulsiveness, inattention, and hyperactivity. Although ADHD is an early developmental disorder, it may persist into adulthood, resulting in deficits associated with poor academic performance, frequent job changes, poor and unstable marriages, and increases in motor vehicle accidents. Of the three primary symptoms of ADHD, deficits in impulse control are the most challenging to the social network and the judicial system. While the etiology of ADHD remains unknown, recent work suggests that the central deficits in ADHD may be due to poor response inhibition that is linked to monoamine and prefrontal lobe deficiencies. In the past, preclinical studies designed to understand the lack of impulse control have generally been relegated to studies linked to aggression and drug abuse. With the use of innovative noninvasive techniques, like anatomical and functional magnetic resonance imaging, selective neurochemical and behavioral paradigms have converged with preclinical reports and lend support to the premise that monoaminergic neurotransmitter systems and the cortico-striatal circuitry are essential to impulse control. Furthermore, new emerging data on neural substrates underlying impulsivity have incorporated brain regions involved in reinforcement, reward, and decision making such as the nucleus accumbens, cerebellum, and amygdala. As noninvasive brain imaging, neurochemical, and behavioral approaches are combined, our knowledge of the neural networks underlying impulsivity will hopefully give rise to therapeutic approaches aimed at alleviating this disorder.