Although participation in 12-step programs is now widely utilized as a treatment aftercare resource by individuals with drug and alcohol problems, little is known about the effectiveness of such a practice. This paper identifies gaps in the existing literature and articulates methodological concerns that may compromise investigations of 12-step programs. It highlights the need for additional after-treatment studies, and it presents findings from a 24-month longitudinal after-treatment study that suggests the effectiveness of 12-step programs. Rather than a behavioral indicator of recovery motivation or a spurious relationship confounded by additional treatment, aftercare, or alumni activities that occur simultaneously with 12-step participation, the findings suggest that weekly or more frequent 12-step participation is associated with drug and alcohol abstinence. Less-than-weekly participation is not associated with favorable drug and alcohol use outcomes, and participation in 12-step programs seems to be equally useful in maintaining abstinence from both illicit drug and alcohol use. These findings point to the wisdom of a general policy that recommends weekly or more frequent participation in a 12-step program as a useful and inexpensive aftercare resource for many clients.