Because 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent chemical carcinogen evaluated by the U.S. Environmental Protection Agency (EPA), many people fear that exposure to even small amounts of TCDD could lead to serious health effects. Ambient measurements confirm that environmental TCDD contamination is widespread. The public is concerned about TCDD exposure from such diverse sources as municipal solid waste incinerators, pulp and paper mills, and contaminated fish and soil. This paper evaluates several critical issues including: (i) the extent of background contamination; (ii) accumulation in the food chain and the potential for human exposure from ingesting contaminated food items; (iii) the magnitude of TCDD emissions into the U.S. environment, and the relative contribution of various known TCDD sources to the total TCDD load; and (iv) setting environmental standards for TCDD.