Floods cause significant economic damage. In Floods cause significant economic damage. In Europe, hydrological phenomena (floods and landslides) caused more than 140 billion euros of damage between 1980 and 2011. In France, floods have been responsible for around 30 billion euros of damage over the last 25 years. Economic assessment aims to objectivize the choices that lead to prevention expenses. It determines the suitability of prevention projects and weighs benefits and costs. It also makes it possible to compare projects on the scale of the territory, if all the studies respect common specifications. Today, economic assessment is an obligatory process for obtaining subsidies from the French State. This chapter therefore deals with the following question: how has the economic assessment of flood prevention projects in France developed over the last decade, from 2007 to 2017?