Using peer sociometric data collected on a large, international (U.S. and Canada) sample of clinically diagnosed 7 to 9 year-old Attention-Deficit/Hyperactivity Disordered (ADHD) children, a detailed analysis of the peer problems of ADHD children will be conducted. The participants will be a subgroup of children participating in the NIMH collaborative Multimodal Treatment Study of Children with ADHD (MTA) at each of three time points (ADHD ns = 171, 301, and 252, at baseline, 14 months, and 24 months, respectively) and their classmates (1,487 total children at baseline; 2,555 at 14 months; and 2,258 at 24 months). A developmental psychopathology perspective will be employed in that the ADHD children will be compared to their classmates in the natural school environment on indices of peer rejection, dyadic friendships, peer network measures, and exploratory measures of social acuity. In addition, peer variables will be considered as predictors, moderators and mediators of treatment response and follow-up outcomes. Using this approach, the following specific aims will be addressed: (1) to comprehensively describe the peer problems of ADHD children in order to better understand the nature of ADHD children's social difficulties, and to identify peer variables that may serve as important risk or protective factors (moderators); (2) to examine how state-of the-art, empirically-supported treatments for ADHD (medication, behavior therapy, combined treatment) compare to each other and to community standard treatments in terms of alleviating the peer problems of ADHD children; (3) to examine the extent to which peer variables assist in the prediction of who will respond to each of several treatments for ADHD; (4) to examine the extent to which peer rejection and deviant peer association, persisting at treatment termination, constitute risk factors for more serious problems at a subsequent follow-up assessment; (5) to consider whether involvement with a deviant peer network just prior to early adolescence (when children are aged 9-11) explains (mediates) the relationship between ineffective parenting and child oppositionality at prior time points, and later conduct problems, delinquency, academic achievement, and substance use. By addressing these goals using an existing data set, they can be accomplished for a small fraction of the cost of a new study.