 This study aims to provide novel phenotypes for genomic studies into Attention- Deficit Hyperactivity Disorder ADHD, one of the most common and heritable of all neuropsychiatric disorders. It proposes to split the disorder into neurobiologically more meaningful entities by delineating subgroups based on neurobehavioral profiles. It will also explore factors that impact clinical course, focusing on the neural effects of treatment and the role of the child s social environment. 