Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by developmentally inappropriate levels of hyperactivity, impulsivity, and/or inattention. ADHD is chronic, may persist into adulthood, and is associated with impairment in social and academic/work domains across the lifespan. Children and adolescents with ADHD often present with executive function deficits and emotion dysregulation, and these deficits may increase impairment and risk for co-occurring disorders. The etiology of ADHD is not yet understood, though research suggests that biological and environmental factors (e.g., family, community) contribute to its development and course. It should be noted that ADHD commonly co-occurs with additional psychiatric disorders, such as oppositional defiant disorder (ODD), conduct disorder (CD), and major depressive disorder. Evidence-based assessment of ADHD requires information from multiple informants using multiple assessment methods to determine the presence of ADHD symptoms across settings and any co-occurring disorders. The evidence-based treatment options for ADHD are manifold. Pharmacotherapy for ADHD is common, although numerous behavioral interventions are also effective. Stimulant medications are commonly prescribed and are typically effective in ameliorating core ADHD symptoms. There is also evidence that the nonstimulant medication atomoxetine substantially decreases the symptoms of ADHD. Importantly, medication therapy works to reduce symptoms but typically does not alleviate the impairments associated with the disorder. Combined medication and behavioral interventions are more likely to reduce impairments and normalize behavior.