Background: To make effective progress towards a global reduction in obesity prevalence, there needs to be a focus on broader structural factors, beyond individual-level drivers of diet and physical activity. This article describes the use of a systems framework to develop obesity prevention policies with adolescents. The aim of this research was to use the group model building (GMB) method to identify young people's perceptions of the drivers of adolescent obesity in five European countries, as part of the EU-funded Co-Create project. Methods: We used GMB with four groups of 16-18-year-olds in schools in each of the five European countries (The Netherlands, Norway, Poland, Portugal and the UK) to create causal loop diagrams (CLDs) representing their perceptions of the drivers of adolescent obesity. The maps were then merged into one, using a new protocol. Results: Two hundred and fifty-seven participants, aged 16-18 years, engaged in 20 separate system mapping groups, each of which generated 1 CLD. The findings were largely congruent between the countries. Three feedback loops in the merged diagram particularly stand out: commercial drivers of unhealthy diets; mental health and unhealthy diets; social media use, body image and motivation to exercise. Conclusions: GMB provides a novel way of eliciting from young people the system-based drivers of obesity that are relevant to them. Mental health issues, social media use and commercial practices were considered by the young people to be key drivers of adolescent obesity, subjects that have thus far had little or no coverage in research and policy. ; The CO-CREATE project received funding from the European Union’s Horizon 2020 research and innovation programme for Sustainable Food Security under grant agreement No. 774210. The content of this article reflects only the authors’ views and the European Commission is not liable for any use that may be made of the information it contains. ; info:eu-repo/semantics/publishedVersion