Strategies to improve health behaviour and well-being of children are needed. A multicomponent intervention “We Act – together for health” was developed with the aim to improve the dietary habits, physical activity, well-being and social capital among school children aged 10-12 years by increasing their health competences and promoting a healthy school environment. This paper describes the theoretical frame for the project, the development of the We Act intervention and the design and methods used in the efficacy study. We Act builds upon the health promoting school approach and the IVAC model. The intervention included three components: 1) An educational component compromising: a) Lunch meal habits integrated into science and Danish (“IEAT”), and physical activity integrated into maths (“IMOVE”); b) Vision workshop integrated primarily in Danish, and; c) Action and Change process at class and school level. 2) A school component including a workshop to develop teachers’ competencies, and 3) A parental component: including homepage, an APP, a Facebook-group and a handout produced by their child. A quasi-experimental study with 4 intervention schools and 4 matched control schools was conducted. In total 656 school children participated. The intervention was carried out in between the baseline and follow up measurements and a process evaluation was carried out to study implementation fidelity. Dietary intake during the school day was measured using a digital photographic method, pedometers registered physical activity, and an electronic questionnaire was used to assess well-being and social capital among the pupils. We Act will provide new knowledge on the implementation and effectiveness of a multicomponent school based intervention targeting school children, teachers, school management and parents to promote dietary habits, physical activity, and well-being among school children. An important part of the evaluation will be to sum up the various intervention components to inform stakeholders and health planning administrators.