Early policies to reduce the amount of toxic waste in the environment focused on cleaning up downstream sources of pollution, such as toxic disposal sites. Public attention in the 1980s encouraged both industry and government to develop an alternative to this command-and-control approach. This article describes the emergence of that alternative-pollution prevention-and its application in Massachusetts through the 1989 Toxics Use Reduction Act. Pollution prevention focuses on the sources of pollution, both metaphorically and physically, more upstream than its predecessors. The success of the Toxics Use Reduction Act in Massachusetts helped create an opportunity where an alternative pollution prevention paradigm could develop. That paradigm, the precautionary principle, is popular among environment activists because it focuses further upstream than pollution prevention by calling attention to the role the social construction of risk plays in decisions regarding the use of hazardous substances. The authors examine the evolution of the precautionary principle through an investigation of three major pathways in its development and expansion. The article concludes with a discussion of the increased potential for protecting public health and the environment afforded by this new perspective.