Objective: This study investigated substance use in rape and child molestation. We hypothesized that perpetrator substance use would be associated with a greater increase in rapists’ aggression in sexual crimes compared with that of child molesters. We also predicted that victim substance use would be negatively related to both rapists and child molesters’ aggression in sexual crimes. Method: The sample included 245 male rapists and 273 male child molesters who had been evaluated at the Massachusetts Treatment Center (MTC) between 1959 and 1991 for potential civil commitment. Data were obtained from offenders’ archival records and were coded by trained research assistants on perpetrator and victim substance use and perpetrator aggression in sexual crimes. Results: Analyses showed that the magnitude of the positive association between perpetrator alcohol use and aggression in sexual crimes did not differ between rapists and child molesters. In contrast, perpetrator drug use was associated with increased aggression among child molesters only. Victim substance use was related to increased aggression among rapists only. Conclusions: The results indicate that victim substance use and perpetrator drug use, but not perpetrator alcohol use, are differentially related to the aggression of rapists and child molesters in sexual crimes. Those findings imply that substance use may play different roles depending on offender type.