Abstract: As many treaties and statutes emphasize, some risks are distinctive in the sense that they are potentially irreversible or catastrophic; for such risks, it is sensible to take extra precautions. When a harm is irreversible, and when regulators lack information about its magnitude and likelihood, they should purchase an "option" to prevent the harm at a later date - the Irreversible Harm Precautionary Principle. This principle brings standard option theory to bear on environmental law and risk regulation. And when catastrophic outcomes are possible, it makes sense to take special precautions against the worst-case scenarios - the Catastrophic Harm Precautionary Principle. This principle is based on two foundations: an appreciation of people's failure to appreciate the expected value of truly catastrophic losses; and an understanding of the distinction between risk and uncertainty. The Irreversible Harm precautionary Principle must, however, be applied with a recognition that irreversible harms are sometimes on all sides of social problems, and that such harms may be caused by regulation itself. The Catastrophic Harm Precautionary Principle must be applied with an understanding that in some cases, eliminating the worst-case scenario causes far more serious problems than it solves. The normative arguments are illustrated throughout with reference to the problem of global warming; other applications include injunctions in environmental cases, genetic modification of food, protection of endangered species, and terrorism.