11.000 people exit prison in Belgium yearly, facing 2 interwoven challenges: reintegrating into society and achieving desistance from crime. While research highlights the transformative potential of peer programs in addressing these issues, critical gaps remain in understanding how, for whom and under what circumstances these programs can create meaningful change. Therefore, PIP aims to investigate the outcomes of peer programs in prison, the mechanisms that drive or inhibit these outcomes, and the contextual factors that shape both success and failure. PIP will produce cutting-edge theoretical and practical insights, equipping academics and professionals with a radically new understanding of how peer programs can reshape the prison experience and support reintegration and desistance. Rooted in realist science, the project starts by developing initial program theories through realist reviews, expert interviews, and participant observations abroad (WP1). In the participatory action research phase, 4 Living labs are co-created, each of which will be rigorously evaluated through a multi-method framework that combines qualitative methods (e.g. observations, ripple effect mapping) with quantitative tools like surveys (WP2). WP3 uncovers generative mechanisms and contextual factors across Living labs, resulting in a refined middle-range theory about peer programs in prison. Through co-creation with stakeholders, PIP produces 7 utilization key end results: 4 innovative peer programs, overview report of (inter)national inspirational practices and innovations, co-creation and implementation model with mechanisms and contextual factors, process and outcome evaluation toolkits for professionals and prisoners, evidence-based policy recommendations, and educational materials for schools. The co-creation process ensures that outcomes are not only innovative but also deeply aligned with the practical needs and aspirations of those who will implement and benefit from them.