Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder (i.e., with onset in the developmental period) involving significant impairment caused by persistent symptoms of inattention and/or hyperactivity-impulsivity. ADHD is significant to consider in the context of abnormal and clinical psychology, given its high prevalence rate (an average of 5% among children, with estimates as high as 12%, persisting among at least 2.5% of adults), its lifelong trajectory, and its high rates of comorbidity with other mental health disorders. This entry covers the symptoms necessary for a diagnosis of ADHD as well as brief information on prevalence rates, associated features, comorbidity, and differential [Page 336]diagnosis. The entry concludes with an overview of the evidence-based assessment used to diagnose ADHD.