Construction projects are filled with uncertainty. Such projects are becoming more complex: Many different interrelated aspects that are subject to change play a role in the delivery of construction work. This makes it interesting to know what exactly is uncertain in these projects, and what can be done to manage uncertainty. The aim of this research is to explore what aspects of complex construction projects are uncertain and what project management teams of public clients do to manage uncertainty before it impacts projects. Four complex construction projects were studied at two large public clients in the Netherlands: Rijksvastgoedbedrijf and Rijkswaterstaat. A total of 17 interviews were carried out across the project teams of these cases. A literature study was conducted to provide a theoretical basis of uncertainty in complex projects and approaches for the management of uncertainty. Uncertainty can be present in eight aspects: Objectives, methods, market, resources, stakeholders, contract, technology and environment. And managing uncertainty can be done in three ways: Learning, representing complexity and selectionism. The results show a variety of uncertainties present in those projects: all eight aspects were found to involve uncertainty in the cases. What especially appears as a challenge is the state of objects in renovation projects. This was a significant uncertainty that played a role in three renovation cases. Interestingly, project teams know before the start of the project that this is an uncertainty, but still face surprises during the execution phase. In addition, objectives, stakeholders and environment were uncertain in most of the cases. Given this broad spectrum of uncertainties, what do project teams do to prepare for this? Public clients mainly use approaches that represent the complexity of the project. They do this mainly by using organizational structures that focus on collaboration and integration. Organizational structures are utilized to facilitate joint risk management between the contractor and the public client. In addition, other approaches are used to represent complexity. Uncertainties are identified and discussed in a project development process to develop new project-specific knowledge. Decision-makers are often involved to increase the decision power of the project team or to escalate matters that are beyond influence of the project team. And lastly, project teams often focus on financial reserves and creating cohesion between project parties to prepare for unforeseen situations. learning is the second most used approach to prepare for uncertainty. It is mainly used in the renovation projects that were studied, where experiments at the site, use of mockups off site and pilots are introduced to learn about the interaction between the construction method, the object and the environment. The findings show that public clients manage a variety of uncertainties through the representation of complexity in the project organization and by learning. Selectionism was not used in the cases, and it is not clear to which extent selectionism is viable in complex construction projects. Based on the literature, it is recommendable for public clients to use integrated project teams to manage complex construction projects.