To clarify the pathophysiological background of attention deficit/hyperactivity disorder (AD/HD) and its comorbidity with oppositional defiant/conduct disorder (ODD/CD) neuropsychological and electrophysiological correlates were investigated. AD/HD cannot be fully explained by an inhibition-specific deficit. Impairments of attentional orienting, response preparation, and response execution processes are implicated as well. Motor response control was particularly impaired in AD/HD+ODD/CD, while AD/HD-only was more impaired in attentional orienting and motor preparation. A dysregulation of central noradrenergic networks is likely. AD/HD+ODD/CD rather represents a separate pathological entity, which differs from both AD/HD-only and ODD/CD-only, than an additive co-occurrence of AD/HD and ODD/CD.