The precautionary principle tells us that we may have reason to take preventive action against a potential danger even if there is scientific uncertainty whether the danger is real or not. This approach is uncontroversial in most safety applications, but has turned out to be controversial in the area of chemical safety. This is also the area in relation to which the principle is most often discussed. The precautionary principle was formulated in the 1970s and 1980s. It has been incorporated into European law and is also endorsed in several international treaties on environmental protection. This chapter discusses how the principle is defined, how it is related to theories of scientific evidence, its limitations, and its relation to other safety principles.