Abstract: In the domain of national security, many people favor some kind of Precautionary Principle, insisting that it is far better to be safe than sorry, and hence that a range of important safeguards, including widespread surveillance, are amply justified to prevent loss of life. Those who object to the resulting initiatives, and in particular to widespread surveillance, respond with a Precautionary Principle of their own, seeking safeguards against what they see as unacceptable risks to privacy and liberty. The problem is that as in the environmental context, a Precautionary Principle threatens to create an unduly narrow view screen, focusing people on a mere subset of the risks at stake. What is needed is a principle of risk management, typically based on some form of cost-benefit balancing. For many problems in the area of national security, however, it is difficult to specify either costs or benefits, creating a severe epistemic difficulty. Considerable progress can nonetheless be made with the assistance of four ideas, calling for (1) breakeven analysis; (2) the avoidance of gratuitous costs (economic or otherwise); (3) a prohibition on the invocation or use of illicit grounds (such as punishment of free speech or prying into people’s private lives); and (4) maximin, which counsels in favor of eliminating, or reducing the risk of, the very worst of the worst-case scenarios. In the face of incommensurable goods, however, the idea of maximin faces particular challenges.