ABSTRACT The Precautionary Principle posits that, in the absence of certainty, the appropriate course of action is to err on the side of caution. The Principle has been applied to decision making and policy development related to environmental health issues both internationally and in the United States. The American Public Health Association and the American Nurses Association (ANA) have issued policy statements that invoke the Precautionary Principle, and the Principle has been incorporated into statements that describe the practice of public health nursing. Nursing has always recognized the relationship of the environment with the health of humans—individuals, families, populations, and communities (ANA). The increasing attention to the Precautionary Principle comes at a time of redefinition of the field of public health, environmental public health, and the practice of public health nursing. Thus, it is crucial that practicing public health nurses understand the Precautionary Principle and its relevance to the practice of public health, public health nursing, and the current and future health individuals, families, populations, and communities.