Some researchers have suggested that Attention-Deficit/Hyperactivity Disorder is currently the most prevalent childhood psychiatric disorder; estimates range between 3 percent and 5 percent for school-age children and 50 percent to 75 percent for clinic-referred children. While there is not yet a cure, psychostimulant treatments ameliorate behavioral symptomatology in 70-80 percent of children with ADHD, depending on how response is defined. Although Methylphenidate (MPH) is the most widely used of the CNS psychostimulants, the neural mechanism through which MPH achieves its effects are largely unknown. We believe that it results in increased cortical arousal that may be measured by the QEEG metric. This study proposes to examine the psychostimulant effects on brain electrical activity among children with ADHD during cognitive activation. The QEEG data will then be subjected to an algorithm developed by Leuchter and associates to estimate the cerebral perfusion underlying the electrode sites. Finally, the relationship between measures of brain activity and improvements in cognitive performance will be examined.