This overview of the role played by the precautionary principle in international trade law, European law and national law compares how precautionary considerations have been applied in the fields of pesticide regulation and the regulation of base stations for mobile telephones in Sweden, the UK and the US. A number of problems in the current application of the precautionary principle are identified and discussed. For example, it is shown that a firm reliance on a wide and open-ended precautionary principle may lead to problems with the consistency, foreseeability, effectiveness and efficiency of measures intended to reduce environmental or health risks. It is suggested that the precautionary principle indeed may be an important tool, but that in order to be acceptable it must be coupled with strong requirements on the performance of risk assessments, cost/benefit analyses and risk trade-off analyses.