ObjectivesThe present review aimed to assess the quality, content and evidence of efficacy of universally delivered (to all pupils aged 5–16 years), school-based, mental health interventions designed to promote mental health/well-being and resilience, using a validated outcome measure and provided within the UK in order to inform UK schools-based well-being implementation.DesignA systematic review of published literature set within UK mainstream school settings.Data sourcesEmbase, CINAHL, MEDLINE, PsycINFO, PsychArticles, ASSIA and Psychological and Behavioural Sciences published between 2000 and April 2016.Eligibility criteriaPublished in English; universal interventions that aimed to improve mental health/emotional well-being in a mainstream school environment; school pupils were the direct recipients of the intervention; pre-post design utilised allowing comparison using a validated outcome measure.Data extraction and synthesis12 studies were identified including RCTs and non-controlled pre-post designs (5 primary school based, 7 secondary school based). A narrative synthesis was applied with study quality check.1ResultsEffectiveness of school-based universal interventions was found to be neutral or small with more positive effects found for poorer quality studies and those based in primary schools (pupils aged 9–12 years). Studies varied widely in their use of measures and study design. Only four studies were rated ‘excellent’ quality. Methodological issues such as small sample size, varying course fidelity and lack of randomisation reduced overall study quality. Where there were several positive outcomes, effect sizes were small, and methodological issues rendered many results to be interpreted with caution. Overall, results suggested a trend whereby higher quality studies reported less positive effects. The only study that conducted a health economic analysis suggested the intervention was not cost-effective.ConclusionsThe current evidence suggests there are neutral to small effects of universal, school-based interventions in the UK that aim to promote emotional or mental well-being or the prevention of mental health difficulties. Robust, long-term methodologies need to be pursued ensuring adequate recording of fidelity, the use of validated measures sensitive to mechanisms of change, reporting of those lost to follow-up and any adverse effects. Further high-quality and large-scale research is required across the UK in order to robustly test any long-term benefits for pupils or on the wider educational or health system.