This article aims at appraising the relationships between environmental risks, their perception by the actors and the measurements which are really taken faced with these risks. It questions environmental risk therefore under the angle of its measure, definition and perception. It confronts then the answers proposed by the actors with theories and practices of collective action. To analyse more in details the relationships between environmental risk and collective action, we first come back on the ambiguities raised by the notion of environmental risk to better understand the problems at stake. This better understanding allows us to focus on the theme of collective action, which reappears again and again when studying environmental risk management. We propose a methodology for studying collective action, which focuses on the role of public policy in the mechanisms of environmental risk prevention and management. Finally, we present our analysis of the public action with respect to this risk over the last several years, concentrating on the erosion risks along the Opal Coast (France).