Precautionary principles have been proposed as a fundamental element of sound risk management. Their advocates see them as guiding action in the face of uncertainty, encouraging the adoption of measures that reduce serious risks to health, safety, and the environment. Their opponents may reject the very idea of precautionary principles, find specific principles unacceptably vague or see them as clearly doing economic damage—either to society as a whole or to their own interests. This article traces the development of alternative precautionary principles, primarily in Europe. Their adequacy is considered in one context where such principles have often been invoked, using genetically modified organisms (GMOs) in agriculture. Although some precautionary principles can be given analytical rigor, the concerns that they express strain the intellectual and institutional structure of conventional policy analysis. © 2002 by the Association for Policy Analysis and Management.