Project SoL is implemented over a period of four years with the aim to promote healthy eating and physical activity among children aged 3–8 years by targeting the families in a Danish municipality based on the multi-component, supersetting strategy. Interventions are implemented in childcare centres, schools and supermarkets in three local communities as well as in local mass media and social media during a 19 months period in the Municipality of Bornholm. The matching Municipality of Odsherred serves as a control site based on its similarity to Bornholm regarding several socio-demographic and health indicators. The present paper gives an account of the design used for the summative and formative evaluation based on a realistic evaluation and a mixed methods approach combining qualitative and quantitative methods. Summative studies are conducted on changes of health behaviours among the involved families and within the municipalities in general, changes in community awareness of the project, changes in purchase patterns, changes in overweight and obesity among the targeted children and changes in knowledge and preferences among children due to sensory education workshops. The formative research comprises studies on children’s perceptions of health, perceptions of staff at supermarkets and media professionals on their roles in supporting the health promotion agenda, and motivations and barriers of community stakeholders to engage in health promotion at community level. The paper discusses operational issues and lessons learnt related to studying complex community interventions, cross-disciplinarily, interfaces between practice and research and research capacity strengthening; and suggests areas for future research. The development and implementation of the intervention and its theoretical foundation is described in a separate paper.