The following study assesses the relationship between affiliating with delinquent peer groups, participation in delinquency, and several substance misuse and injecting drug use outcomes in a nationally representative sample of inmates in state and federal facilities in the United States. After controlling for potential confounders, affiliating with peers who engaged in deviant behaviors and participation in delinquency was associated with onset of alcohol and illicit drug use, substance dependence, alcohol dependence, types of substances used, and injecting drug use outcomes. Inmates who began engaging in delinquency at older ages reported initiating drug and alcohol use at older ages, and were less likely to meet the criteria for drug abuse or dependence, less likely to use substances daily or near daily, and less likely to report having ever injected or shared syringes. The implications of these findings for substance abuse, HIV, and crime prevention interventions are discussed.