The goal of this chapter is to present a review of research into attentional aspects of hyperactivity disorder. Before starting with the review, some issues, which are not mutually exclusive, will be discussed. The issues concern the appropriateness of the paradigms commonly used to study attention and the accuracy of the methods of identifying hyperactivity in children. Many attempts have been made to validate the clinical concept of hyperactivity disorder in the laboratory. However, since the manifestation of the primary symptoms of hyperactivity (inattention, impulsivity and overactivity) largely depend on the types of environments in which the child is observed, the laboratory approach may be criticized because of the limited range of situations in which the child's behaviour is studied. In addition, experimental task demands may be highly sophisticated, so that extensive training is needed before the actual experiment can start. More over, in order to keep the child ‘on the track’, he or she is commonly not left to perform the task alone. When all these factors are taken into account, it could be argued that the symptoms of hyperactivity may have been ‘filtered out’ by the time the laboratory experiment starts. Consequently, one may question the extent to which an attention deficit in hyperactivity disorder, as indicated by teachers and parents and defined by clinical diagnostic procedures, can be accounted for by laboratory data. Barkley (1991) examined the degree of ecological validity of laboratory experiments of hyperactivity.