Abstract: When catastrophic outcomes are possible, it makes sense to take precautions against the worst-case scenarios — the Catastrophic Harm Precautionary Principle. This principle is based on three foundations: an emphasis on people’s occasional failure to appreciate the expected value of truly catastrophic losses; a recognition that political actors may engage in unjustifiable delay when the costs of precautions would be incurred immediately and when the benefits would not be enjoyed until the distant future; and an understanding of the distinction between risk and uncertainty. The normative arguments are illustrated throughout with reference to the problem of climate change; other applications include avian flu, genetic modification of food, protection of endangered species, and terrorism.