Noel Weiss discusses the limitations of epidemiologic studies, outlining under what circumstances an etiologic factor cannot be identified through nonexperimental studies of individuals. This lecture was taped at the 2004 Epidemiology, Biostatistics and Clinical Research Methods Summer Session co-sponsored by the Seattle VA Epidemiologic Research and Information Center (ERIC) and the University of Washington. From the Series:Epidemiology: Introduction to Epidemiologic Methods