Rigorous reviews of the science on the effectiveness of Alcoholics Anonymous (AA) indicate that AA and related 12-step treatment are at least as helpful as other intervention approaches. Exactly how AA achieves these beneficial outcomes is less well understood, yet, greater elucidation of AA's mechanisms could inform our understanding of addiction recovery and the timing and content of alcohol-related interventions. Empirical studies examining AA's mechanisms were located from searches in Pubmed, Medline, PsycINFO, Social Service Abstracts and from published reference lists. Thirteen studies completed full mediational tests. A further six were included that had completed partial tests. Mechanisms examined fell into three domains: (1) Common processes; (2) AA-specific practices; and (3) Social and spiritual processes. Results suggest AA helps individuals recover through common process mechanisms associated with enhancing self-efficacy, coping skills, and motivation, and by facilitating adaptive social network changes. Little research or support was found for AA's specific practices or spiritual mechanisms. Conclusions are limited by between-study differences in sampling, measurement, and assessment time-points, and by insufficient theoretical elaboration of recovery-related change. Similar to the common finding that theoretically-distinct professional interventions do not result in differential patient outcomes, AA's effectiveness may not be due to its specific content or process. Rather, its chief strength may lie in its ability to provide free, long-term, easy access and exposure to recovery-related common therapeutic elements, the dose of which, can be adaptively self-regulated according to perceived need.