The precautionary principle states that preventive measures should be taken against a serious potential danger, even if the existence of the danger has not been fully confirmed by science. As codified in European legislation and international treaties, the precautionary principle is science-based, since it requires scientifically reasonable suspicions of danger. A wide range of scientific disciplines need to be involved in the application of the precautionary principle. This entry argues against two deviations from science-based precaution. One of these is to let any type of suspicion of danger trigger precaution, even if it has no base in science. Such a practice would give influence to conspiracy theorists and proponents of pseudoscience. The other deviation is to do nothing against a potential danger unless there is full scientific evidence of its existence. This standpoint is glaringly implausible, but risk-imposing industries such as the tobacco, chemical and fossil fuel industries have spent large resources on propagating it as an essential part of their scientific denialist messages.