Background: In 2016 The Belgian government launched a Masterplan to improve healthcare for Mentally Ill Offenders (MIO). The Plan aims at improving collaboration between different organizations to guarantee integrated healthcare trajectories. The core question of this contribution is to assess to what extent the policy framework and guidance facilitates or hampers the implementation of collaboration and integrated care for MIO’s. Methods: Qualitative data were collected using document analysis of the Reform program and semi structured interviews with representatives of hospitals, justice houses and regional coordinators. Data-analysis was done with a continuous comparative method of coding and analysis including researcher triangulation.Results: The Masterplan, the general reference text on the Reform, mandates locoregional steering groups to implement the reform. These groups are supported by formally appointed and funded regional coordinators. Locoregional actors get leeway to implement the reform within the overall framework.The interviews learn that intersectoral collaborations are needed to tackle problems on offering healthcare for MIO. Implementation problems emerge from the heterogeneous composition of steering groups and diverging interests, power struggles and differences in culture. Some participating organizations focus on forensic thinking while others have a broader overall mental health care approach. The steering groups need to find a balance between mutual interdependence and organizational interests. Historical partnerships versus integrating new services in this MIO-field challenge the definition of common goals and working practices. Moreover participating organizations are represented by different professional backgrounds (managers vs clinical professionals) which affects the view on the problem and collaboration. At last, the interactions are affected by the regulations of the partners from different policy sectors.Discussion: Heterogeneity and complexity challenges collaboration. Policy ...