The precautionary principle has been hailed as the new paradigm for contending with health and environmental risk in the context of emerging technologies. In the philosophical literature, however, it has been met with skepticism. Weaker conceptions of the precautionary principle are accused of being trivial or vacuous, while stronger versions are criticized for issuing irrationally restrictive or even contradictory prescriptions. Although the precautionary approach suffers from a number of conceptual defects, it nonetheless could be justified in certain biological domains if it were the case that evolution tended to produce optimal, delicately balanced equilibria that generally coincided with what we value. This justification fails, however, since it is premised on assumptions about the causal structure of the world that do not accord with contemporary evolutionary theory. This does not exclude the possibility that the precautionary principle may be warranted for other reasons or in certain settings, but it does remove a potentially powerful rationalization, one that has motivated much of the scholarship, law, and policy that is inclined toward the precautionary approach.