The reintegration of offenders is an important social problem, with an increased investment in programs to support offenders in the reintegration process. From the academic disciplines of social work and sports sociology, four different knowledge gaps concerning the reintegration of prisoners become apparent: 1) The lack of knowledge from an insider perspective, which means that knowledge on the perspective of offenders, an important element in the development of meaningful interventions, is lacking. 2) A lack of knowledge on several life domains which can contribute to the re-integration of offenders, such as the domain of sports. 3) The lack of research outside of the criminological discipline, reducing the notion of reintegration to the prevention of recidivism. 4) A lack of knowledge in sports sociology to create new fundamental theoretical insights about how organized sports can both act as an inclusive space and as a vehicle for broader integration. In this research we address these gaps from a social work theoretical perspective on social re-integration of offenders. Using the methodology of a realist approach we develop through a multiple case study of 5 sport programs a middle range program theory on the working mechanisms of sport programs in prison in relation to the different dimensions of social re-integration, seen from a social work research perspective.