The global obesity epidemic is a multifactorial problem associated with severe health consequences. The lifestyle behaviours of diet and physical activity are learned early, often tracking into adulthood. Identifying preventive interventions to establish children’s healthy lifestyle behaviours, in line with recommended guidelines, is a public health priority. The ‘Food Dudes’ programme is a well-evidenced behaviour change intervention, producing large and lasting increases in children’s consumption of fruit and vegetables in primary school settings. An extension of the programme, targeting physical activity, is the novel ‘Dynamic Dudes’intervention, recently trialled in primary schools. This thesis aimed to develop and evaluate a ‘Super Dynamic Food Dudes’ intervention, underpinned by the same behavioural principles as the respective primary school programmes. Chapter 1 provides a detailed literature review and identifies a paucity of such interventions. Chapter 2 modified and evaluated the Food Dudes Early Years intervention designed to increase pre-school children’s consumption of fruit and vegetables. To identify a suitable activity measurement tool, Chapter 3 validated use of the consumer-grade Fitbit Zip accelerometer to measure pre-school children’s step counts. Chapter 4 and Chapter 5 describe the development and feasibility of two physical activity components, interactive stories and in-class exercise DVDs, respectively, which were later integrated in a controlled pilot of the new physical activity intervention in Chapter 6. Drawing on outcomes and process evaluation from the preceding chapters, Chapter 7 describes and justifies further modifications to the interventions. Chapter 8 presents a controlled evaluation of the finalised two-pronged multi-component intervention in the school setting, with extension to the home. Main short-term outcomes were large and significantly greater increases in consumption of fruit and vegetables, and physical activity, in the intervention conditions, compared to the control conditions, replicating findings in primary school children. The pre-school multi-component intervention provides a promising method of preventing childhood obesity.