Few developments in risk management have created as much controversy as the Precautionary Principle (PP), which generally prescribes caution when managing environmental risks in the face of scientific uncertainty. The PP is, by the Treaty on European Union, the keystone of European environmental policy, and has been invoked in European decisions about regulation of exposure to electromagnetic fields, and entry of genetically modified crops and beef from hormone-fed cattle into European markets. However, the PP lacks clear definition, and its different statements in various international treaties have greatly different implications. This talk will describe the PP as it has been used in the European Community, and some of the problems and controversies that have arisen about it. A recent commentary by the European Union offers much-needed guidance how to apply the PP in protecting against environmental and human health risks.