As reviewed in Chapter 3 of this volume, extensive data are now available that suggest that individuals with attention deficit/hyperactivity disorder (ADHD) exhibit differences in transporter gene markers that strongly suggest a genetic predisposition for functional brain differences. Neuroanatomical and neurophysiological scans consistently reveal that individuals with ADHD exhibit abnormalities in the size and level of activity on the frontal and prefrontal lobes, basal ganglia, and cerebellum. These differences are associated with underactivation of dopaminergic and noradrenergic neurotransmission that underlies the difficulties in self-control, impulse control, and sustained attention that individuals with ADHD exhibit.