ABSTRACT. Objective: Medication-assisted treatment for substance use disorders (SUDs) is not widely used in treatment programs. The aims of the current study were to document the prevalence of adoption and implementation of extended-release injectable naltrexone, the newest U.S. Food and Drug Administration-approved medication for alcohol use disorder (AUD), in U.S. treatment programs and to examine associations between organizational and patient characteristics and adoption. Method: The study used interview data from a nationally representative sample of 307 U.S. SUD treatment programs to examine adoption and implementation of injectable naltrexone. Results: Thirteen percent of programs used injectable naltrexone for AUD, and 3% of programs used it for opioid use disorder. Every treatment program that offered injectable naltrexone to its patients used it in conjunction with psychosocial treatment, particularly cognitive behavioral therapy. Multivariate logistic regression results indicated that adoption was positively associated with the provision of wraparound services, the percentage of privately insured patients, and the presence of inpatient detoxifi cation services. For-profi t status and offering inpatient services were negatively associated with adoption. Within adopting programs, an average of 4.1% of AUD patients and 7.1% of patients with opioid use disorder were currently receiving the medication, despite clinical directors' reports of positive patient outcomes, particularly for relapsers and for those who had been noncompliant with other medications. Cost was a signifi cant issue for the majority of adopting organizations. Conclusions: The rate of adoption of injectable naltrexone in U.S. treatment programs remains limited. Researchers should continue to examine patient, organizational, and external characteristics associated with the adoption and implementation of injectable naltrexone over time. (J. Stud. Alcohol Drugs, 76, 143-151, 2015)