In its role as a public program and the single largest payer of behavioral health services in the country, Medicaid is particularly well positioned to implement policy to improve the delivery, quality, and effectiveness of behavioral health services. Our analysis finds that 7.3 percent of Medicaid enrollees ages 12 to 64 had at least one clinically-identified substance use disorder in 2019, but this is likely an undercount, as other research suggests that prevalence is at least 4 times higher. People with clinically-identified SUD were more likely to be male, White, over 25 years old, and qualify for Medicaid based on a disability or through Medicaid expansion. Rates of clinically-identified SUD vary across states not only because of prevalence, but also because of other factors, such as provider screening behavior and variation in Medicaid coverage of services. National recommendations instruct providers to screen for substance use and conduct brief interventions for adults 18+, yet there may be gaps between SUD screening and referral. Other factors--such as patient privacy concerns or few healthcare visits--may also play a role in low identification of SUD.