Malnutrition, characterised by the imbalanced intake of nutrients or impaired nutrient utilisation, is a leading health risk factor among children globally. The double burden of malnutrition among children is prevalent in Asia including Thailand. The effectiveness of school nutrition interventions in reducing this problem in Asia remains unclear. Limited information exists regarding why and how school nutrition interventions are effective/ineffective. This lack of evidence hinders the advancement of nutrition policy in Asia. My PhD research assessed the impact and factors influencing the impact of school based nutrition programmes on over- and undernutrition in school-aged children in Asia, focused specifically on Thailand. I conducted a systematic review and meta-analysis to examine school nutrition interventions in Asia. I subsequently did impact and process evaluations of a complex school nutrition intervention in Thailand, using a quantitative analysis of the data obtained from a quasi-experiment and qualitative methods, respectively. My meta-analysis indicated that school nutrition interventions in primary schools across Asia focused primarily on and were effective in reducing Body Mass Index (BMI) and BMI-for-age z-scores. The impact evaluation of the intervention in Thailand demonstrated lowered risks of overweight and obesity and an increase in height-for-age Z-scores among young school children, but no significant impact on wasting. My process evaluation revealed perceived strengths, which included its multi-component design and its multi-sectoral support system to facilitate the implementation of the intervention. Perceived limitations were the lack of specific services for addressing wasting and suboptimal fidelity. The findings also underscored the importance of having supportive education and health policies and establishing an enabling school context (i.e., policy, leadership, capacity, external support, social and physical environments). My thesis suggests that complex school nutrition interventions are effective in reducing overnutrition and more evidence regarding the impact on undernutrition is needed in Asia. In Thailand, the intervention has the potential to reduce overnutrition and stunting; however, specific services for addressing undernutrition are required. Additionally, supportive national policies and school contexts are crucial for ensuring effective implementation and maximum impacts.