There is strong evidence that the catecholamines dopamine and norepinephrine are both important in the pathophysiology of ADHD, as well as in the mechanism of therapeutic action of stimulant drugs. Due to the known effects of stimulants in blocking reuptake of catecholamines and (in the case of D-amphetamine) facilitating their release, it has traditionally been believed that the stimulants compensate for catecholamine deficiency in ADHD. More recently, however, alternate hypotheses of a hyperdopaminergic and/or hyper-noradrenergic state in ADHD have been suggested. This paper will be limited to a review of the evidence for involvement of dopamine in mediating behavioral and cognitive symptoms and response to stimulants in ADHD, with implications for possible mechanisms.