The precautionary principle has often been described as an extreme principle that neglects science and stifles innovation. However, such an interpretation has no support in the official definitions of the principle that have been adopted by the European Union and by the signatories of international treaties on environmental protection. In these documents, the precautionary principle is a guideline specifying how to deal with certain types of scientific uncertainty. In this contribution, this approach to the precautionary principle is explicated with the (...) help of concepts from the philosophy of science and comparisons with general notions of practical rationality. Three major problems in its application are discussed, and it is concluded that to serve its purpose, the precautionary principle has to (1) be combined with other decision principles in cases with competing top priorities, (2) be based on the current state of science, which requires procedures for scientific updates, and (3) exclude potential dangers whose plausibility is too low to trigger meaningful precautionary action. (shrink)