The precautionary principle holds that we should not allow scientific uncertainty to prevent us from taking precautionary measures in response to potential threats that are irreversible and potentially disastrous. Critics of the principle claim that it deters progress and development, is excessively risk-aversive and is unscientific. This paper argues that the principle can be scientific provided that the threats addressed by the principle are plausible threats, and the precautionary measures adopted are reasonable. The paper also argues that one may use (...) epistemic criteria, such as consistency, coherence and explanatory power, to determine whether a threat is plausible, and that one may use practical considerations, such as effectiveness, proportionality, cost-effectiveness, realism and consistency, to determine whether a response to a threat is reasonable. (shrink)