Background: Mindfulness-based therapy (MBT) has become a popular form of intervention. However, the existing reviews report inconsistent findings. Objective: To clarify these inconsistencies in the literature, we conducted a comprehensive effect-size analysis to evaluate the efficacy of MBT. Data sources: A systematic review of studies published in journals or in dissertations in PubMED or PsycINFO from the first available date until May 10, 2013. Review methods: A total of 209 studies (n = 12,145) were included. Results: Effect-size estimates suggested that MBT is moderately effective in pre-post comparisons (n = 72; Hedge's g = .55), in comparisons with waitlist controls (n = 67; Hedge's g = .53), and when compared with other active treatments (n = 68; Hedge's g = .33), including other psychological treatments (n = 35; Hedge's g = .22). MBT did not differ from traditional CBT or behavioral therapies (n = 9; Hedge's g = −.07) or pharmacological treatments (n = 3; Hedge's g = .13). Conclusion: MBT is an effective treatment for a variety of psychological problems, and is especially effective for reducing anxiety, depression, and stress.