Cognitive biases have been theorized to play a critical role in the onset and maintenance of anxiety and depression. Cognitive bias modification (CBM), an experimental paradigm that uses training to induce maladaptive or adaptive cognitive biases, was developed to test these causal models. Although CBM has generated considerable interest in the past decade, both as an experimental paradigm and as a form of treatment, there have been no quantitative reviews of the effect of CBM on anxiety and depression. This meta-analysis of 45 studies (2,591 participants) assessed the effect of CBM on cognitive biases and on anxiety and depression. CBM had a medium effect on biases (g = 0.49) that was stronger for interpretation (g = 0.81) than for attention (g = 0.29) biases. CBM further had a small effect on anxiety and depression (g = 0.13), although this effect was reliable only when symptoms were assessed after participants experienced a stressor (g = 0.23). When anxiety and depression were examined separately, CBM significantly modified anxiety but not depression. There was a nonsignificant trend toward a larger effect for studies including multiple training sessions. These findings are broadly consistent with cognitive theories of anxiety and depression that propose an interactive effect of cognitive biases and stressors on these symptoms. However, the small effect sizes observed here suggest that this effect may be more modest than previously believed.