The negative social, educational and health consequences of obesity for children have been well documented, with downstream impacts on chronic disease for adults. A diet high in energy-dense nutrient-poor foods, and low in nutritious foods, is an important contributor to obesity. Such diets tend to have negative health and educational outcomes for children. Childhood nutrition can be considered a complex system, with many influences on children's diets across home, community and school settings. Schools are often identified as a site for interventions to promote healthy diets. However, given the complexity of influences, the impact of school based actions is likely to be limited. The first aim of this study was to identify a 'portfolio' of interventions across school, home and community settings that, taken together, will support primary schools to effectively promote healthy nutrition. Informed by complexity theory, a policy research method of analysis was developed. The research method sought to combine an understanding of case study primary school food environment 'systems', with the views of policymakers. Testing the research method for use in policy analysis of complex issues was the second aim of this study. The food environments of five case study primary schools within the Wellington region of New Zealand were mapped using interview, documentary and observational data. Intervention options to improve the school food environment were identified across case studies, with support for interventions gathered from school principals. Interviews with sixteen policymakers considered the national level context of interventions. To inform implementation, identified interventions were prioritised based on: (i) the level of support from case study school principals and policymakers; (ii) evidence of effectiveness from international literature; and (iii) theoretical likelihood of impacting on the complex system of childhood nutrition. The top identified priority was to encourage schools to develop food policies that would promote consumption of healthy foods and minimise unhealthy foods within the school. Such school policies can be supported with external expertise, nutrition focused health promotion programmes such as Fruit in Schools, and policy settings that direct schools to consider nutrition issues. Second and third priorities focus on home and community environments and include restricting food marketing to children, increasing the affordability of healthy foods, and social marketing campaigns. Comprehensive actions across policy settings are required for effective healthy nutrition promotion within primary schools. Overall, the method proved to be useful for identifying intervention options to address complex policy issue. A manageable portfolio of interventions was identified to work across the systems under study. A number of tensions were evident within the complexity theory informed research method used for this study. These tensions included: balancing central government planning with flexibility at the community level; capturing enough information to adequately understand the entire 'child nutrition system'; and the degree to which a 'system perspective' challenges the current machinery of government. In conclusion, improving child nutrition is a significant public policy issue. Taking a complexity theory perspective to policy research and analysis aided development of a portfolio of interventions to impact widely across the 'system', from which child nutrition practices 'emerge'. Taken together the interventions are likely to act to support schools to improve their food environments and effectively promote healthy nutrition. The results are a starting point for detailed policy design. This research suggests that the analysis method used deserves further investigation and refinement. The test for policymakers will be to develop cost effective interventions, which take account of local complexities, within a government system that favours linear programme logic and accountability lines.