Attention-Deficit/Hyperactivity Disorder (ADHD) [1] is a seemingly heterogeneous group of cognitive/behavioural developmental disorders where all clinical criteria are behavioural. ADHD affects between 2% and 12% of grade-school children [2-5]. The disorder usually, but not always, manifests itself before the child is 7 years old [6,7]. Of children diagnosed with ADHD, 50-70% will have problems related to social adjustment and functioning, and/or psychiatric problems as adolescents and young adults [8,9]. The primary aim of the ADHD group at the Norwegian Centre for Advanced Study was to combine behavioural and neuroscientific basic research on Attention-Deficit/Hyperactivity Disorder (ADHD). The group represented the first, broad interdisciplinary collaboration between clinical researchers and basic researchers on ADHD anywhere in the world. The international interdisciplinary group had as its major objective to try to reach a thorough understanding of ADHD. To a large extent this was obtained. All participants contributed to the new ways of looking upon ADHD. The group obtained more than it was reason to expect in advance. Part of the reason for this is the high quality of the group members, the fact that most of them were able to spend several months at CAS, and the careful, long-term planning (about 2 years) before the group gathered for the first meeting.