Precaution is a key issue in environmental governance. Variously defined, intensively debated and introduced in many regulations, its meaning, scope and application remain problematic. This article argues that the controversy on precaution is a matter of culturally patterned expectations concerning the production and use of knowledge and the related social positions and responsibilities. The way uncertainty and its role in the policy process are understood is crucial. For some precaution is a flawed concept, to be accommodated to the current expert-based cooperative scheme. For others it is a major innovation requiring a rearrangement of the latter. Precautionary policies may evolve in different directions. They may either strengthen the role of means-ends rationality, increasing people's dependence on expert knowledge and shrinking the opportunity and scope of public debate or, on the contrary, enhance the role of value-commitments, leading to a decline in the legitimacy of established hierarchies and an intensification of intractable controversies.