"Western societies devote substantial resources to media health campaigns, especially targeting young people. Unfortunately, health campaigns often have disappointing impact. There is a compelling need for insight into campaign implementation in social life. Especially among youth, the social environment plays a crucial role in message processing. Therefore, health campaigns can only be improved with a multidisciplinary approach that can predict youth’s behaviour within their social environment.The proposed programme aims to develop a method for effective campaign implementation via youth’s social networks. This method will (1) identify and target the most powerful influence agents in youth’s social networks, (2) tailor campaign messages to the most effective influence mechanisms, and (3) evaluate the effectiveness of social network implemented campaigns. The method will be based on the multidisciplinary Media X Social Influence (MXSI) model, integrating theories from communication, marketing, psychology, and sociology.Five research projects focus on youth’s weight-related behaviours. Drawing on a large-scale cross-sequential cohort study (N = 3,000, 9-15 y/o), the research (Phase I) tests and refines the MXSI model, identifying social influence agents and mechanisms, and (Phase II) tests the effectiveness of a social network-implemented health campaign in a group-randomised control trial, targeting the most powerful influence agents and mechanisms. The research projects use the Wearable Lab, a highly innovative smartphone-based research methodology.The project is theoretically and methodologically pioneering and will produce a unique and useful method for improving health campaigns. Feasibility and success are guaranteed by the applicant’s extensive experience and network. The applicant is one of the leading experts in the world on the role of marketing communication in youth socialisation and in translating these insights for health and prosocial purposes."