We analyzed survey data from Canada, Chile, Germany, The Netherlands, and the United States to study the prevalence and treatment of mental and substance abuse disorders. Total past-year prevalence estimates range between 17.0 percent (Chile) and 29.1 percent (U.S.). Many cases are mild. Although disorder severity is strongly related to treatment, one- to two-thirds of serious cases receive no treatment each year. Most treatment goes to minor and mild cases. Undertreatment of serious cases is most pronounced among young poorly educated males. Outreach is needed to reduce barriers to care among serious cases and young people at risk of serious disorders.