This master thesis is developed based on the desire of the water and sewage department in the municipality of Karlskrona. The department, which vouches for the quality of produced drinking water and sewage cleaning, was in need of help dealing with risk analysis. The water and sewage department is obliged to identify and assess risks according to Swedish legislation. However, no guides for this purpose regarding safety, health, environment and quality were available. The aim of this report was to accomplish a guideline for the performance of risk analysis within water and sewage plants. If the guideline is used, the requirements in the legislation will be obtained. Even other advantages can be expected. A risk analysis helps identifying needs for improvement. This might lead to a better working environment and a more reliable water and sewage system.