Childhood intoearly adolescenceis a critical developmental periodfor mental health, with growing evidence that universal school-based wellbeing interventions can improve children’smental health outcomes, with lifelong benefits. The A Lust for Life (ALFL) school-based positive psychology intervention is widely implemented in Ireland, with someevidence of benefits. Thiscluster randomized controlledtrial (RCT)aimed to evaluatethe effectiveness of the ALFL programme for children in fifthand sixthclass grades, aged 9-13 years, using a measure of state wellbeing as the primary outcome variable, with trait measures of wellbeing, depression and anxiety as secondary outcome variables, building on previous researchstudies. Participants were 402 children, recruited from 9schools, randomly allocatedto a 10-week ALFL programme group(n = 180)or10-week waiting list control group(n = 222).Outcome measures were administeredpre and post intervention/waiting time. The trial showed that theALFL schools programmeled to small improvements in statewellbeing arising from using behavioural skills learned on the ALFL programme(d = 0.18, p =.034), and an increase in the use of skills learnt on the programmeto promote state wellbeing (d = 0.27, p = .001), butno significant improvements in measures of trait wellbeing, anxiety or depression.This study’s use of a state wellbeing measureand its considerationof children’s real-worlduseof skills learnt, which arecentral tothe evaluated intervention’s theory of change, constitutean original contribution to the research base.