Individuals with ADHD have a high rate of comorbid psychiatric disorders, especiallysubstance use disorders. Similarly, ADHD is overrepresented in the SUDpopulation. This high rate of comorbidity can make ADHD difficult to diagnoseand treat. Comorbid SUD in individuals with ADHD can have a negative impact oncourse of illness and quality of life. The stringent DSM-IV criteria for ADHDmay make diagnosing ADHD in adults difficult, which may lead to anunderdiagnosis of ADHD in the adult population. This may be especially true forpatients with SUD, because cognitive deficits associated with substance abusecan hinder their ability to recall ADHD symptoms for appropriate diagnosticpurposes. On the other hand, SUD symptoms may mimic ADHD symptoms, which canlead to an overdiagnosis of ADHD in the SUD population. If proper attention ispaid to age-appropriate symptoms of ADHD, and careful longitudinal data areobtained from patients presenting with ADHD or SUD, proper treatment can begiven to patients with these comorbid disorders.