Hazardous substances in the environment can affect human health in a number of ways. For example, some environmental contaminants are suspected or known carcinogens, while others are associated with vascular disease, reproductive toxicity or endocrine disruption. Health Effects research includes the following major areas of study: identification of the causative agent(s), determination of the minimum dose where adverse health effects manifest, development of diagnostic tools for detecting chemical agents in biological systems, and explanation of the mechanisms by which these substances cause toxicity. The new knowledge acquired in these studies can be incorporated into the risk assessment of hazardous waste sites, as well as strategies for preventing adverse human health effects.