This article presents findings from a meta-analysis which sought to determine the effectiveness of interventions adopting a whole school approach to enhancing children and young people's social and emotional development. Whole school interventions were included if they involved a coordinated set of activities across curriculum teaching, school ethos and environment, and family and community partnerships. A total of 45 studies (30 interventions) involving 496,299 participants were included in the analysis. Post-intervention outcomes demonstrated significant but small improvements in participants' social and emotional adjustment (d = 0.220), behavioural adjustment (d = 0.134), and internalising symptoms (d = 0.109). Interventions were not shown to impact on academic achievement. Origin of study and the inclusion of a community component as part of a whole school approach were found to be significant moderators for social and emotional outcomes. Further research is required to determine the active ingredients of whole school interventions that we can better understand the components necessary to achieve successful outcomes.