ADHD continues to be one of the most common psychiatric disorders in children and is recognized increasingly as a common psychiatric disorder in adults. Diagnosis of the disorder requires careful consideration of other psychiatric and medical disorders that may mimic symptoms of ADHD. Comorbid disorders are common in patients with ADHD and require careful attention. A recommended diagnostic evaluation for children and adults includes the completion of a psychiatric interview, rating scales from multiple informants, and often individually tailored psychologic testing. Pharmacologic treatment options include the first-line stimulant agents for adults and children and TCAs and atypical antidepressants as second-line agents. Behavioral interventions may help reduce ADHD symptoms and address comorbid conditions in children. Future research is needed to identify the optimal psychosocial treatment for adults with ADHD.