This article adduces evidence of the central role played by scientists in the 1970s and “lay persons” in the post-Chernobyl period in the production and legitimation of alternative types of knowledge and expertise on the environmental and health risks of nuclear energy in France. From a constructivist perspective, it argues that this shift in the relationship of “lay persons” to knowledge production is linked not only to the rise of mistrust vis-à-vis scientific institutions but also, and especially, to a change in the way they have reacted to “dependency” on institutions and to “state secrecy”. Counter-expertise is constructed as a politics of surveillance where alternative interpretations of risk are buttressed by a permanent critique of the epistemic assumptions of institutional expertise. The identity of “counter-expert” is socially elaborated within this process.