This paper is the text of a lecture given to an audience composed, for the most part, of students of chemical engineering at a major university. The objective of the paper is to inform such a young, intelligent, and highly technical audience of the basic facts and perceptions--scientific, technical, societal, regulatory, and legal--concerning environmental health hazards. Topics such as toxicity, hazard, and risk--and ways in which risk has been approached and perceived--are discussed as are various public and private efforts to reduce the risks of environmental health hazards. The great uncertainties encountered in this field and seriousness with which it must be taken--including a brief description of possible personal legal liabilities an engineer can face--are stressed. Despite all difficulties, progress has been made in bringing order to a chaotic field and more progress is both needed and possible.