Drug treatment aftercare is frequently cited as necessary for individuals served within the criminal justice system. The purposes of this article are to review how much is actually known about aftercare and to highlight issues in studying the role of aftercare. We begin with a review of the literature, looking at how aftercare is defined within the criminal justice system outcome literature and the findings on aftercare for offenders who received initial treatment from in-prison substance use treatment programs. We continue with a discussion of how substance use treatment provided within the federal system, drug use patterns, and responses to drug use create methodological difficulties in adequately assessing the effectiveness of aftercare services. Taking into account both the previous research on aftercare and the issues encountered in attempting to evaluate the federal aftercare services, we concluded that the claim of certainty about aftercare effectiveness is not well substantiated and that the precise nature of aftercare services needed is not well understood. We conclude with a discussion of the methodological and substantive issues that need to be addressed in future research. Issues identified include the need to address self-selection bias and to disentangle offender behavior from the effects of criminal justice system policies. Research is also needed to identify the most effective type and intensity of aftercare.