Project portfolio management (PPM) refers to a set of processes and practices to manage a group of projects and programs to achieve strategic business objectives. The focus of project portfolio management until now has been on project selection and prioritization and on the strategic alignment of projects. The current literature on project portfolio management makes little mention of potential disturbances to project portfolios such as new projects, terminated projects, delayed projects, incorrect planning due to high uncertainty, and changes in external environment. This is somewhat surprising considering that management in the face of uncertainty has been studied for a number of years in the fields of change management of single projects, organization theory, and strategy theory. This research investigates the following research question: How is uncertainty affecting project portfolios managed in dynamic environments? An uncertainty management perspective is adopted instead of the more established practice risk management. While different approaches have been developed in the context of the management of single projects, these ideas have not been carried over to the management of project portfolios. It is unclear whether these approaches are applicable at the portfolio level or whether it is sufficient to incorporate flexibility at the project level to gain flexibility at the portfolio level.