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This is a review of current relevant evidences concerning the nature and pathophysiological mechanisms of attention deficit hyperactivity disorder (ADHD). From a neuropsychological point of view, clinical symptoms seem to arise from an early dysfunction of the executive system. Patients with ADHD have deficits in inhibitory control, cognitive flexibility, working memory and self-motivation, and all of them account for the attentional deficit in non automatic information processing. Decrease in prefrontal, caudate and pallidal structures, which sustain the executive function, have been found in neuroimaging volumetry. Cognitive evoked potentials obtained during attentional tasks have augmented latencies and abnormal topography. A dopaminergic deficit in the structures sustaining executive function is postulated from the results in experimental animal models and from functional neuroimaging studies in patients, and this seem to be the foundation of the favorable outcome with psychostimulants in correctly diagnosed patients. Psychopedagogic interventions are necessary to help the patient in order to get an optimal internal locus of control, which is necessary for attention and impulsiveness inhibition, and also for compensation of associated disorders.