This article reviews and synthesizes information about the relationship between two distinct approaches to helping substance users, formal "addiction treatment" and 12-step mutual aid. The following issues are addressed: Extent of formal treatment and 12-step fellowship participation for persons with substance use-related problems; overlap between treatment and 12-step participation; early attempts to "integrate" treatment and 12-step mutual aid; differences and similarities between the 12-step program and treatment; issues of spirituality in the 12-step program; effectiveness of 12-step participation as treatment aftercare; and whether help-seekers can be "matched" to 12-step. The article poses pertinent questions that could answered by additional research, including life cycle patterns of usage of treatment and 12-step, outcomes of such episodes, reasons for using different interventions at different times, feasibility of 12-step participation as a primary intervention, the relative contributions of striving for spiritual values vs. social support factors to the effectiveness of 12-step, and the comparative effectiveness of secular vs. 12-step mutual aid. The article concludes with a proposed reconceptualization of the relationship between formal treatment and 12-step mutual aid that may help in structuring future research.