The challenges in trying to achieve population-level lifestyle changes have seen researchers and policymakers shift towards a focus on environmental improvements to support health-related behaviours. School environments are dynamic food and policy environments that have the capacity to change both the risks and protective factors for healthy eating and physical activity in children. The essential focus of this thesis has been to explore childhood-obesity prevention policies at a country level, and to explore the increasing problem of obesogenicity in primary school settings in Brunei Darussalam.To achieve this, three methods were adopted: a policy analysis of nutrition and physical activity initiatives, targeting the prevention of obesity in primary school settings among children aged 8 to 11 years old in Brunei, Malaysia and Singapore; case studies in five primary school settings; and a stakeholder analysis, providing insight into the perspectives of those with an interest in children's health, including policymakers, teachers, school administrators, and the children themselves.The key research findings are that the Brunei Darussalam's childhood-obesity prevention response can be strengthened by increasing the attention to upstream approaches, using school settings and school neighbourhoods, and that school community members including school administrators and teachers can play an important role in contributing to the obesity-prevention programs in the school settings, but are currently under resourced to play this role. Further, continuing to primarily blame parents for their children being overweight and obesity is a limited way of understanding the problem and is a barrier to policy innovation and community acceptance of environmental measures.The outcomes of this study are intended to benefit students and school community members through improved country-level and school-level policies that create environments to support dietary and physical activity behaviours in children. Specific policy recommendations for Brunei Darussalam include a more systematic approach to data collection, monitoring and surveillance of childhood obesity, as well as a strengthening of evidence-based obesity prevention at the environmental level.My research makes an important contribution to knowledge in the field by adopting a more holistic approach to policy research examining the various aspects of the built and food ii environments and the policies that support these environments. New methods are proposed for describing, analysing, and improving environments that are important settings for obesity prevention; and the importance of engaging key stakeholders as leaders of change is highlighted.iii