The YoPA project, ‘a youth-centred preventive action approach towards co-created implementation of socially and physically activating environmental interventions’ obtained funding from Horizon Europe’s Health Cluster. The project addresses the multifaceted challenges of physical inactivity and health inequalities through a unique participatory approach. The project places teenagers between 12 and 18 years old in vulnerable situations at the forefront of the intervention process. The Institute of Tropical Medicine is a partner in the project and will conduct a Realist Evaluation to understand how youth co-creation contributes to improved adolescent health and well-being in four cities in Denmark, Netherlands, Nigeria and South Africa.  By integrating its results and sharing its approach in an open access Toolbox, ITM aims to contribute to fostering sustainable, youth-led solutions for healthier urban environments.