The worldwide obesity epidemic is a major and complex problem, resulting from the interaction of individual metabolic, genetic, and psychological factors with meso- and macro environmental factors. Adolescents, with their rapid changes in body composition, together with their shift in orientation from the direct family to the peer-related environment, are particularly vulnerable to unhealthy changes in life style. Habits with respect to food and physical activity tend to change substantially in this period as compared to earlier childhood, and peers seem to play an important role in shaping the behavior of adolescents. Adolescents tend to mimic their peers' behavior related to both healthy and unhealthy food choices as well as to physical activity and sports participation. This influence of peers depends on environmental circumstances such as availability of snacks, and is affected by individual psychological and demographic factors, and by macro-environmental factors such as cultural and social norms, legislation, food distribution, and media and advertising. A rapidly expanding source of influence on health behavior are media such as television, the internet (e.g., Facebook, twitter), and smartphones. These media may enhance overweight and reduce physical strength by reinforcing a sedentary life. However, potentially these media could be used to monitor and influence snacking behavior and physical activity for the better. An example of the use of new technology to enhance health behavior is the 'Wii' home video game. Using insights from social psychology, thoughtful choice architecture can be established to nudge adolescents to choose what is best for them without restricting freedom of choice and to help them with really implementing healthy intentions. A challenge for the coming decade is to develop 'Apps' aimed to decrease adolescent obesity while making use of social networks.