Increasing international concern is being expressed regarding the contamination of the environment with polychlorinated dibenzo-p dioxins (PCDDs) and polychlorinated dibenzofurans because certain of these chemicals have been shown to be highly toxic to animals and are ubiquitous in the environment. They are known to be distributed as contaminants of commercial products and as by-products from com bustion processes. A considerable volume of information has accumulated on these chemicals in the past two decades, particularly for the most toxic of them, 2,3,7,8-tetrach1orodibenzo-p-dioxin (2,3,7,8-TCDD). However, this body of knowledge has not succeeded in resolving genuine judgmental differences among experts in the field as to the degree of hazard to human health and the environment. In light of the widespread public concern, it is clearly imperative to come to grips with the continuing scientific controversy, to review the data, assess the issues, to see where areas of agreement exist, and where further research is needed to resolve remaining areas of disagree ment. This volume represents an effort to contribute to these goals.