Purpose: Complexity is seen as the main bottleneck to start re-use processes for vacant heritage buildings. Researchers ask for the development of conceptual frameworks to explore project complexity and for tools to manage these complex processes. Aim: This research focuses on collaborative complexity and aims firstly to provide an understanding of the complexity within the collaboration in heritage re-use projects and secondly to reduce this complexity with a clear overview on the role and responsibility divisions of stakeholders in the process. Research question: How can a clear overview on role and responsibility divisions contribute to a reduction of the complexity of heritage re-use processes? Methodology: The complexity is explored by means of a literature study, four empirical case studies including a cross-case analysis and a focus group discussion. The collaboration within empirical heritage re-use processes is mapped with the help of: 1) a timeline, which gives an overview on the iterative nature of the process; 2) a process description that explains the key activities mentioned in the timeline; 3) a relation structure explaining how stakeholders are linked to each other and which roles they fulfil; 4) a collaboration framework that explains the role and responsibility division between stakeholders per process step; and 5) a complexity assessment framework explaining which aspects increase the level of complexity. Findings: Complexity of heritage re-use processes cannot be reduced with a clear overview on role and responsibility divisions: These projects are unique, a clear overview on role and responsibility divisions in these types of processes could therefore not be made. The division of roles and responsibilities amongst stakeholders seems to differ within every project and changes occur during the process. Stakeholders cannot anticipate with certainty on changes as the dynamics are unknown at the start and they differ extremely per project. It might however be possible to steer on aspects that increase the level of complexity within the collaboration, or to deal with the complexity by implementing a step-by-step approach. Limitations of the research: Qualitative research is always subject to bias due to the interpretation of the qualitative information. Subsequently, the modest number of samples that is investigated in this master thesis may not adequately represent the target population. Practical implications: This research raises awareness on the complexity of heritage re-use processes and the limited action that is taken up until now to decrease the vacancy among cultural and historical valuable buildings in the Netherlands. It offers recommendations for initiators of heritage re-use processes to deal with the complexity within these processes to increase the amount of re-uses of heritage buildings in practice. Scientific relevance: This research tries to fill gaps in literature on process complexity and collaboration in heritage re-use processes. It argues that heritage re-use processes should not be set in blueprints as these models lack the ability to map the complexity of these processes. Originality/value: Limited literature is available on heritage re-use processes, this research increases the insight in these processes.