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Chronic pain places a burden on individuals and the economy. Although there is evidence for the effectiveness of cognitive-behavior therapy, it is recognized that the effects are limited. Acceptance and Commitment Therapy (ACT), which aims to increase valued action in the presence of pain, has been suggested as an alternative approach. The objective of this review was to determine the clinical effectiveness of ACT for chronic pain in adults when compared with control conditions and other active treatments.The searches of this systematic review were conducted in the Cochrane library, MEDLINE, EMBASE, CINAHL Plus (EBSCO), and PsycINFO. Grey literature, reference list, and reverse citation searches were also completed.Eleven trials were included. ACT was favored over controls (no alternative intervention or treatment as usual). Significant, medium to large effect sizes were found for measures of pain acceptance and psychological flexibility, which are typically considered processes of ACT. Significant small to medium effect sizes were found for measures of functioning, anxiety, and depression. Measures of pain intensity and quality of life were not significantly different than zero. Generally effect sizes were smaller at follow-up.ACT was more clinically effective than controls on a number of outcomes. It is possible that methodological limitations, some of which are common to psychological trials, may have led to overestimated effects. Only a few studies compared ACT to active treatments and while the evidence is promising for ACT in the treatment of chronic pain, further methodologically robust trials are required.