Environment and health refers to those aspects of human health, broadly defined, influenced by the environments (physical, social, biological, psychosocial, chemical, economic, political, cultural) within which we live, work, and play. The disappearance of the fourth largest inland body of water in the world (Aral Sea, Uzbekistan), an as yet inexplicable fourfold increase in asthma in the last two decades in much of the developed world, babies dying of contaminated water in the developed world (Walkerton, Ontario, Canada), hundreds of thousands of babies dying as a result of contaminated water in the developing world, an as yet inexplicable doubling of deadly peanut allergy in most of the Western world, so much waste we cannot safely dispose of it all, such high demands for energy that we imperil human safety through the proliferation of nuclear power; these are all examples of relationships between human health and the environment that are of paramount and increasing importance to researchers, policymakers, and the general public. This situation has been punctuated by the occurrence of major environmental disasters such as Seveso (Italy), Love Canal (New York), Three Mile Island (Pennsylvania), Bhopal (India) and Chernobyl (former USSR). Ongoing concerns about creeping environmental disasters keep environment and health on the front page of the newspaper, and the top of the research agenda. The primary challenge in addressing these relationships involves the complexity of designing investigative studies, measuring exposures and outcomes, and evaluating the evidence. Despite these difficulties, decisions must be made with respect to policy, regulations, and guidelines, put in place to protect the health of the public.