This dissertation was motivated by wide-ranging and longstanding debates on the validity of Attention-Deficit/Hyperactivity Disorder (ADHD) as a disorder. To advance understanding of the construct of ADHD as a disorder we examined two broad questions on which the validity of ADHD rests and which we view as impeding epidemiologic efforts to improve diagnosis, treatment and prevention—what is the natural history of ADHD and how can we examine the validity of ADHD as a disorder? To this end, we applied the harmful-dysfunction framework of disorder to the study of ADHD. First, in a conceptual overview we examined the existing literature on etiology, course and neurobiology to reframe the ADHD validity debate within a harmful-dysfunction frame. We found that although the harm criterion of the harmful-dysfunction framework of disorder is fairly well established, research on the neurobiology of ADHD has yet to elucidate the internal-dysfunction criterion. Many advances have been made in this area but none yet applicable to the diagnostic process. Second, in a community sample of youths followed into adulthood we found that despite a growing perception that ADHD is a lifelong disorder the majority of individuals outgrew ADHD by adulthood and persistence continued to decline with increasing age. Third, to help distinguish symptoms driven by internal-dysfunction from normative responses to disordered environments—a predominant alternative hypothesis to ADHD as a valid disorder—at a macro level we characterized context as potentially eliciting symptoms of ADHD as a normative response. We found that this index of “normalizing context” predicted symptoms from late childhood to early adulthood. Moreover, we provided an example of how the relationship between “normalizing context” and ADHD can be studied in behavioral research to help clarify whether symptoms are due to internal-dysfunction. It is our hope that as a proof-of-concept, this empirical example and its conceptual grounding can spark similar research on the validity of ADHD that may lead to more effective uncoupling of harm from internal-dysfunction at a behavioral level and ultimately advance research on ADHD at all levels of inquiry to improve diagnosis, treatment and prevention.