Introduction: Non-milk extrinsic sugar {NMES} intake among children in Scotland fails to meet dietary recommendations, which is likely to contribute to dental decay and weight gain. Children's physical activity, which contributes to energy balance, also fails to meet recommended targets. Aim: To develop and evaluate a classroom based educational intervention to improve knowledge of NMES and energy balance in children aged 10-12 years. Methods: Following development of an intervention, consisting of two educational sessions, one on NMES and one on physical activity and energy balance, a cluster-randomised trial was conducted with 268 children in 15 primary schools {8 controls}. Children completed questionnaires to assess their knowledge of NMES and energy balance and levels of physical activity at baseline and 4, 10 and 34 weeks post intervention. Intake of NMES was assessed at baseline, and at weeks 10 and 34 using a food frequency questionnaire. After the intervention, focus groups were conducted to explore children's views and opinions of NMES and the educational sessions. Results: At baseline children had limited knowledge of NMES. Post intervention, the intervention group had a significantly higher knowledge than the control group, but this declined by 34 weeks. Knowledge of energy balance between the groups was not significantly different post intervention. No changes in NMES intake or physical activity were observed post intervention. Focus groups highlighted that children did not consider health to be a priority for dietary choice. They thought it was important to learn about NMES, but were disinclined to reduce their intakes. Conclusions: Despite improved knowledge of N MES and the health consequences of excess consumption, children are unlikely to change dietary behaviour when it requires over-riding their taste preferences. Education helps children to understand healthy lifestyle behaviours, but following this, environmental changes may be necessary to facilitate behaviour change.