Building on a detailed study of the Woburn, Massachusetts, childhood leukemia cluster, this paper examines lay and professional ways of knowing about environmental health risks. Of particular interest are differences between lay and professional groups' definitions of data quality, methods of analysis, traditionally accepted levels of measurement and statistical significance, and relations between scientific method and public policy. This paper conceptualizes the hazard-detection and solution-seeking activities of Love Canal, Woburn, and other communities as popular epidemiology: the process by which lay persons gather data and direct and marshal the knowledge and resources of experts in order to understand the epidemiology of disease, treat existing and prevent future disease, and remove the responsible environmental contaminants. Based on different needs, goals, and methods, laypeople and professionals have conflicting perspectives on how to investigate and interpret environmental health data.