Interventions aimed directly at reducing weight or preventing increase in weight shows limited results on long term effects and it is shown that increase in activity has more impact on health outcome than on weight-loss. Further, weight-loss does not seem to be an appropriate measure of therapeutic interventions for growing children. The hypothesis of this study is that focusing on increasing meaningful physical activity through individual tailored counselling will have positive effects on the adolescents' fitness (health) and quality of life. Thus, the overall purpose of the study is to examine the extent to which a web-based intervention influences physical activity, fitness and quality of life in over-weight and obese adolescents Further, the aim is to explore and describe how adolescents experience being over-weight and obese, what they perceive as meaningful physical activity and finally how adolescents experience lifestyle changes as demanded in the intervention study.