Coastal erosion is a natural process caused by the actions of current and waves towards coastlines. Building with Nature (BwN), is an emerging methodology that utilizes natural process and natural materials to reduce the impacts of coastal erosion on nature and society. Because of the involvement of natural materials and natural processes, BwN proactively embraces uncertainties in the coastal management projects, which deviates from customary methods of coastal management that reduce uncertainties. Therefore, it is meaningful to investigate uncertainties in the BwN projects and how uncertainties influence decision making processes of the BwN projects. This master thesis project selects a BwN sand nourishment project in Ystad, Sweden (in short, the Ystad project) for case study. The research aims to analyze uncertainties in the BwN sand nourishment projects and identify influential uncertainties in decision making processes of the BwN projects. In order to achieve the research objective, the master thesis project starts with a literature review on existing theories of uncertainty analysis, including different definitions and classifications of uncertainties. Based on previous research, a BwN uncertainty matrix is established to reveal uncertainties and different perceptions of uncertainties. Three dimensions, including the location, level and nature of uncertainty, are used to depict uncertainties in the BwN projects. This master thesis project applies the BwN uncertainty matrix to analyze the data collected by desk research and the field research in Sweden. Uncertainties exist in all the natural, technical and societal systems in the decision making process of the Ystad project. Uncertainties regarding issues in the societal system are addressed most frequently, while technical uncertainties are rarely raised by respondents. Different perceptions of uncertainties, particularly regarding environmental impacts of sand nourishment and the availability of sand resource for sand nourishment are essential and affected the decision making process. Ystad Municipality, as project owner and local authority in Ystad, made a lot of efforts to break through the delayed decision making process by providing knowledge, persuasive communication and interactive scale framing. The main contribution of the master thesis is developing an actor-based BwN uncertainty matrix to identify uncertainties and compare different perceptions of uncertainties in the BwN projects. It is also the first time to have a qualitative uncertainty analysis for the first sand nourishment project in Sweden.