Objective: ADHD is often comorbid with other disorders, but it is often assumed that academic, language, or motor skills problems are secondary to ADHD rather than that attention problems are secondary to the other disorder or both disorders have a shared etiology. We assessed for comorbid developmental disorders and which cognitive processes were impaired in children with ADHD. Method: Measures of intelligence, language, motor skills, social cognition, and executive functions were administered to children with ADHD (n = 53) and age/sex-matched typical children. Results: Clinically significant deficits were 2 to 7 times as common in children with ADHD as in typical children, and the structure of ability differed in the two groups. Abilities were less differentiated in children with ADHD. Conclusion: The results indicate a need for comprehensive screening for developmental disorders in children with ADHD and imply that research needs to focus on how ADHD and developmental disorders may share an etiology. (J. of Att. Dis. 2012; XX(X) 1-XX)