This systematic review of systematic reviews addresses a current gap in evidence by synthesizing findings relating to weight management interventions set entirely in primary/elementary schools targeting, diet, and/or physical activity as key strategies.Eight databases were searched for systematic reviews of trials of school-based interventions targeting children aged 4 to 12 years that looked at biometric and behavioral outcomes. From the 10 selected systematic reviews, we found that interventions designed to promote physical activity or reduce sedentary behavior were most effective for weight loss. Interventions designed to improve diet and nutrition had a small effect on behavioral and cognitive outcomes, and these outcomes could be enhanced through the use of experiential learning. The most effective interventions involved a range of stakeholders in the development process and included parents and families in implementation. This systematic review of systematic reviews offers evidence-based guidance for the development and implementation of multistrategy weight-management interventions in primary/elementary schools. K E Y W O R D S Childhood obesity, primary/elementary schools, systematic review 1 | INTRODUCTION Being overweight in childhood is associated with poorer quality of life 1 and excess morbidity and mortality in adulthood. 2 Research in the UK shows that BMI has been increasing among children entering reception class (aged 4/5) and accelerating through to the last year of primary school (year 6, aged 10/11 years). 3 This suggests that intervention with school-aged children should take place as early as possible, and the primary school setting offers a key opportunity to intervene early, at a crucial time in children's development. Early intervention has the potential to have short and long-term effects on a number of health, educational, and social outcomes 4 and to reduce health inequalities globally. 5 The aim of the present research was to review systematically what is known about the effectiveness and contents of primary/elementary school-based interventions to prevent or treat excess weight gain.Weight-management interventions are increasingly being evaluated in school settings, and subsequently the amount of research published on this topic, including reviews of reviews, has increased. 6,7 It is difficult to draw firm conclusions from these previous reviews of reviews on three grounds. First, they have generalized across children from early childhood to adolescence, at all stages of their schooling, 8 which is problematic because BMI naturally increases with age. 1 Second, previous reviews of reviews have focused on specific obesityprevention single strategies such as increasing physical activity in