Over the coming decades, the increasingly popular precautionary principle is likely to have a significant impact on policies all over the world. Applying this principle could lead to dramatic changes in decision making. Possible applications include climate change, genetically modified food, nuclear power, homeland security, new drug therapies, and even war.We argue that the precautionary principle does not help individuals or nations make difficult choices in a non-arbitrary way. Taken seriously, it can be paralyzing, providing no direction at all. In contrast, balancing costs against benefits can offer the foundation of a principled approach for making difficult decisions.