[Page 787]Epidemiological research indicates that comorbidity, or disorder co-occurrence, is quite common among psychological disorders, particularly among children and adolescents. Co-occurrence of psychological disorders (e.g., attention-deficit/hyperactivity disorder [ADHD] with conduct disorder [CD], CD with depression, depression with anxiety disorders) presents tremendous challenges for assessment, course, prognosis, and treatment. Compared with having only one disorder (e.g., ADHD), the presence of a comorbid condition (e.g., CD, depression) is associated with increased symptom severity for each condition, increased impairment in multiple domains, more frequent and severe negative correlates and sequelae, differential treatment response, and distinct courses and developmental pathways. As individuals age, prevalence rates for single and comorbid conditions decline; however, conditions that remain in adulthood are often linked to increasing severity. Given the high prevalence rates of ...