Attention-deficit/hyperactivity disorder (ADHD) is an impairing but usually treatable condition. Popular culture propagates the myth that ADHD recedes with age; this is not the case. Although it is common, <20% of adults with ADHD are diagnosed or treated. Adults with ADHD show significant comorbidities with depressive disorders, anxiety disorders, substance use, oppositional defiant disorder, personality disorders, sleep problems, and learning disabilities. However, symptoms that result from ADHD, such as mood symptoms or lability, are often mistaken for comorbid disorders. Comorbidity with ADHD impacts treatment compliance, treatment response, and patient insight. Insufficient data on the interaction between ADHD and comorbidities impedes proper diagnosis and treatment. Better clinical tools for assessing these conditions are needed. Food and Drug Administration-approved pharmacologic treatments for adult ADHD include stimulants, dexmethylphenidate, and the nonstimulant atomoxetine. Effect sizes of approved medicines at approved doses are half those seen in children. Adults may also need longer duration of medication effects than children. Short-acting stimulants are likely to result in poorer adherence and have a higher risk for diversion or abuse. Risk of abuse is a major concern; stimulant treatments are controlled substances, and children with ADHD show increased risk of substance abuse. Psychosocial interventions may be beneficial in treating both ADHD and comorbidities.In this expert roundtable supplement, Margaret Weiss, MD, PhD, presents a comprehensive overview of complications surrounding differential diagnosis in adults with ADHD. Next, Mark A. Stein, PhD, reviews evaluation, comorbidity, and development of a treatment plan in this population. Finally, Jeffrey H. Newcorn, MD, provides a discussion on the pharmacologic options available for adults with ADHD, considering dosages specific to adults and common comorbidities.