This paper looks at the way in which the idea of the Precautionary Principle, increasingly influential in environmental and other policy areas, is being and might be used in foreign and security policy. It aims to contrast the relative precision with which the term is used in the environmental arena with the current usage in international relations. Contrasting the Precautionary Principle with ideas of precaution, prevention, pre-emption and similar terms in post-structuralist analyses of risk, humanitarian intervention and US foreign policy in the aftermath of 11 September 2001, the paper identifies costs and benefits in deploying a more carefully specified account of the Precautionary Principle. In particular, it highlights key issues of regulatory authority and the way in which policy-makers and analysts understand and respond to the limits of knowledge and knowledge systems as important challenges to which careful use of the Precautionary Principle can potentially contribute. The paper concludes by suggesting that both policy-making and policy analysis could potentially be improved by adapting and extending the idea of the Precautionary Principle as it is deployed in other policy arenas.