Chapter 20—The Prevalence and Severity of Psychiatric Comorbidities provides a summary of a landmark study in epidemiology, the The National Comorbidity Survey Replication (NCS-R). This chapter study sought to answer some fundamental questions. How common are comorbid psychiatric conditions? What are the prevalence and severity rates for comorbid anxiety, mood, impulse control, and substance use disorders? Starting with these questions, this chapter describes the basics of the study, including funding, study location, who was studied, how many patients, study design, study intervention, follow-up, endpoints, results, and criticism and limitations. The chapter briefly reviews other relevant studies and information, discusses implications, and concludes with a relevant clinical case.