Tool No. 25 illustrates how to conduct a Social Impact Assessment (SIA) in order to take into account public perception of risk in flood management plans. After explaining the concept of risk and, in particular, of social risk, the publication describes the factors that influence the perception of flood risk by the population. In so doing, it aims to help flood professionals understand how risk perception determines the reaction of social actors before, during and after a flood event. Different instruments are provided to guide readers in the development of Social Impact Assessments: in the first place, a general methodology that applies to any project having the goal to reduce flood impacts; secondly, a more specific scheme tailored on the scope and objectives of Integrated Flood Management; finally, a grid for the organization of focus groups within the framework of a flood-specific SIA, with the purpose to enhance community participation in the decision-making process.