peer reviewed ; Background: The twelve Integrated Care Program pilot projects (ICPs) created by the government plan ‘Integrated Care for Better Health’ aim to achieve four outcome types (the Quadruple Aim) for people with chronic diseases in Belgium: improved population health, improved patient and provider experiences and improved cost efficiency. The aim of this article is to present the development of a mixed methods realist evaluation of this large-scale, whole system change programme. Methods: A scientific team was commissioned to co-design and implement an evaluation protocol in close collaboration with the government, the ICPs and several other involved stakeholders. Results: A protocol for a mixed methods realist evaluation was developed to gain insights into the mechanisms that foster successful results in ICPs. The qualitative evaluation proposed will be based on the document analysis of yearly ICP progress reports, selected case studies and focus group interviews with stakeholders. Processes and outcomes of all the projects will be monitored using indicators based on administrative data on population health and the quality and costs of care. A yearly survey will be organized to collect data on patient-reported outcomes and experiences and on provider-reported measures of inter-professional collaboration and proper wellbeing. Using both quantitative and qualitative data, we will develop theories about the mechanisms and the associated contextual factors that lead to integrated care and the Quadruple Aim outcomes. Discussion: The objective of this study is to deliver policy recommendations on strategies and best practices to improve care integration in Belgium and to implement a sustainable monitoring system that serves both policy makers and the stakeholders within the ICPs. Some challenges due to the large scale of the project and the multiple stakeholders involved may impede the successful implementation of this proposal. ; FAITH.be - Federated consortium for Appraisal of Integrated care Teams in ...