The Evidence-Base for Positive Psychology Interventions: A Mega-Analysis of Meta-Analyses. Alan Carr1,2, Laura Finneran1, Christine Boyd1, Claire Shirey1, Ciaran Canning1, Owen Stafford1, James Lyons1, Katie Cullen1, Cian Prendergast1, Chris Corbett1, Chloe Drumm1, Tom Burke1,2 1. University College Dublin, 2. Clanwilliam Institute, 3. National University of Ireland Galway This study is a quantitative synthesis of meta-analytic evidence for the effectiveness of very broadly defined positive psychological interventions (PPIs), i.e. interventions that enhance wellbeing through pathways consistent with positive psychology theory. The definition covers a wide range of PPIs including single and multi-element PPI programs as well as mindfulness, mind-body, and physical exercise-based interventions. A mega-analysis of meta-analyses of studies which evaluated PPIs defined in this way was registered with Prospero (CRD42020160217) and conducted. Five databases were searched. 198 meta-analyses involving 4,063 primary studies and 501,335 participants were selected for review. Using AMSTAR-2 criteria, selected meta-analyses were found to be of moderate or high quality. At post-intervention, PPIs had a significant small to medium effect on wellbeing, g = 0.34 [0.29, 0.39], QoL, g = 0.41 [0.37, 0.45], strengths, g = 0.42 [0.39, 0.45], depression, g = 0.42 [0.37, 0.47], anxiety, g = 0.41 [0.34, 0.48], and stress, g = 0.42 [0.36, 0.48]. Gains were partially maintained at 7.5 months follow-up. Mind-body PPIs such as yoga and tai chi were particularly effective. Individuals who engaged in longer, face-to-face programs benefited most. Larger effect sizes occurred in meta-analyses of trials with inactive rather than active control groups and in meta-analyses of lower methodological quality. This mega-analysis shows that PPIs have an extensive evidence base supporting their effectiveness, especially longer, face-to-face PPI programs that incorporate mind-body interventions. Further mega-analytic research on PPI change mechanisms is required.