This chapter synthesises findings from meta-analyses on the effectiveness of Mindfulness-Based Stress Reduction (MBSR) in enhancing mental health among various populations, including healthy adults, veterans, and individuals with diabetes, cancer, chronic medical conditions, and fibromyalgia syndrome. The aim is to establish robust empirical evidence assessing the efficacy of MBSR. It is found that MBSR has large effects on reducing stress, moderate effects on alleviating anxiety, depression, distress, and improving quality of life, and minor effects on reducing burnout among healthy adults. Meta-analyses provide evidence that MBSR is effective in enhancing mental health among veterans and individuals with diabetes, cancer, chronic medical conditions, and fibromyalgia syndrome. The mental health outcomes examined in these meta-analyses include overall mental health, stress, anxiety, depression, post-traumatic stress disorder, sleep quality, and fatigue. It is concluded that MBSR effectively improves mental health across different populations. Based on the synthesis, this chapter also provides directions for future research into the effectiveness of MBSR.