The introduction sets the stage by spotting the controversy about the use of socio-economic analysis in REACH, which is the main European regulation on chemical risks. The use of this tool has allowed many chemicals with known toxicity, in particular carcinogenic, to be maintained on the market. This goes against innumerable alerts about the health effects of toxic chemicals, and systematic methodological criticism brought since the ‘70s to cost-benefit analysis. Why is then socio-economic analysis, and in particular cost-benefit analysis, used in managing chemical risks in Europe? After having shown that academic literature is very limited and does not respond to this question, the chapter introduces the central hypothesis of the book: cost-benefit analysis has been increasingly used in REACH because it has proven to be a convenient tool for a form of risk regulation that I call “regulatory co-management”, and which can be described by six characteristics.