The precautionary principle (PP), as it appears in international treaties or in some countries’ legal systems, suggests that the prospect of scientific progress should not justify the delay of preventive measures. Three effects identified in the economics literature – the irreversibility, the precautionary and the ambiguity aversion effects – may be consistent with the normative content of the PP. A difficult question is how then the PP can be implemented. Several social actors may want to take advantage of a current lack of scientific evidence to promote their own interests. The PP can also be misused, for example, for demagogy or protectionism.