This intervention research project aims to develop and evaluate a school-based intervention to promote physical activity and reduce sedentary behaviours among vocational school adolescents. According to the UK Medical Research Council guidance for complex interventions, we began with an evidence synthesis phase (“Pre-phase”; 2012-13) to systematically review literature and gain insight in the target population using qualitative and quantitative research methods. The findings from this evidence synthesis phase as well as collaboration with stakeholders, experts and target group informed the development of the intervention (Phase I; 2013-14). In the spring 2014 we conducted a small-scale feasibility study to test acceptability and feasibility of central intervention components and other relevant features of the study (Exploratory trial, Phase II; 2014) before the main trial. Currently we are in the process of optimizing intervention content and trial procedures based on the evidence from the feasibility study. This coming winter we will proceed to a definitive cluster-randomized controlled trial to evaluate effectiveness and cost-effectiveness as well as mediating mechanisms of change (Phase III; 2014-17). The future dissemination of the intervention, if found effective, is already being planned with major actors in Finnish vocational education and teacher education.