Editors' Summary: Determining the impact that environmental forces have on human health is an integral part of environmental law and policy. A determination of this impact must, of course, begin with a determination of whether there has been any impact at all. Evaluating the causal relationship between environmental forces and human illness is, therefore, essential. This Article, written by a professor of epidemiology at the University of Alabama at Birmingham's School of Public Health, examines the epidemiologic process for assessing causation, both for purposes of environmental litigation and for purposes of environmental regulation. The Article analyzes three situations in which it is necessary to evaluate causation: an individual research study, an assessment of a general causal hypothesis, and an examination of a specific individual's illness. The Article discusses the criteria for establishing causation and reviews factors, other than causation, that may lead to apparent correlations between exposures and diseases. Finally, the Article suggests ways to approach evaluations of causation to promote more effective use of scientific evidence in environmental law and policymaking.