The lack of regular physical activity is one of the major factors that lead to premature death in western industrialised countries. Intervention programmes are needed that promote an active lifestyle already from a young age. This dissertation thesis contributes to the research in the field of health-promotion through physical activity in young people in school that has been shown to be one of the most suitable settings for such interventions. Based on theoretical considerations from the perspective of sport psychology, sport pedagogy, and training science and on the findings of a systematic review gathering all previous evidence of the effects of health-promoting physical activity interventions in school, HealthyPEP, a health-promotion programme in physical education, was designed and evaluated using a quasi-experimental design with 516 sixth grade students. The degree to which the programme was implemented and the intervention effects on a) the psychological determinants of physical activity, b) the behaviour of physical activity, and c) the health and fitness level were analysed. A positive picture immerged concerning the implementation of HealthyPEP and its evaluation especially by girls. These findings are in line with the positive intervention effects of HealthyPEP on girls motor performance and the positive developments on girls attitudes towards physical activity and knowledge of the health effects of physical activity. Additionally, positive intervention effects were revealed on BMI in both genders. This study confirmed that HealthyPEP improved girls health and that further research is needed to develop programmes that are suitable for boys.