Final Report Summary - MYMOVEZ (Social Network Implementation of Health Campaigns Among Youth) Youth are an important target group for health communication interventions, because they are susceptible in adopting unhealthy habits and behaviors. They are surrounded by social influence agents and role models co-determining their energy balance-related behaviors. Social media enable to reach youth via their social networks. Until recently, research attention was still limited to sporadic and ad hoc evaluation studies and there was a lack of a systematic, comprehensive, and predictive approach. This project aimed to develop a method for effective campaign implementation by targeting young people's social networks via social media. Social forces (i.e. important classmates and friends that influence youth's health behaviors) were used to enhance campaign effects on physical activity, snacking and drinking behavior.The project involved a large-scale cross-sequential cohort study (N = 1,000; 8-15 y/o) and several randomized control trials and smaller experimental studies (pilot studies) which were carried out during a 5-year period. The cohort study has 3 phases that run during a 3 year period. Phase I involved baseline measurements (2016) aiming to identify the most influential social agents and their influence mechanisms, with follow-up measurements one (2017) and two (2018) years later. The project examined youth's media and social network exposure, socialization processes, psychological determinants of behavior, and energy balance-related behaviors with the 'Wearable Lab.' The Wearable Lab is a smart phone-based research application connected to an activity tracker bracelet. In Phase II, campaign message strategies were developed and tested with the influence agents based on the baseline and follow-up data as well as several pilot studies. Finally, two social network-implemented campaigns were tested in Phase II, one focusing on physical activity and one on water consumption.The project is completed, with important results achieved. The Wearable Lab collects data successfully for social network implemented health campaigns while keeping young people motivated to participate in longitudinal research due to its modern and attractive research method. It enables data collection of individual and environmental determinants of energy balance-related behaviours on daily randomized and planned time points, and allows the implementation and evaluation of health campaigns via social media. Findings show that intrinsic motivation and friend's social norms are important determinants of health related behaviours among youth. This suggests that campaign messages should make use of role models and use combined messages framed to intrinsically motivate children while referring to external social norms. The social network interventions that were designed based on these insights, using an autonomy-supportive approach and “vlog” messages, were effective in changing the social norm about the desired behaviours and in improving youth’s responses to the campaign messages, including exposure time, liking, and identification. Social network implementation of the campaigns was more successful than non-targeted “mass media” implementation. However, when it comes to actual behavioural change, the effectiveness of social network intervention depends on the type of health behaviours (dietary versus physical activity behaviours) and how youth perceive the social norms in their peer network before the intervention.