The assessment of risk in human populations relies heavily on the science of epidemiology. Epidemiology is that branch of medicine covering the study of the incidence and transmission of disease in populations. Thus the focus of study is groups of people rather than the individual. There are, as yet, no epidemiological studies that have specifically investigated the public health importance of biofilms in potable water systems. Nevertheless, epidemiological investigation is essential to the understanding of waterborne disease. In 1854, John Snow simultaneously invented the science of epidemiology and proved that drinking water was capable of spreading disease. Since then, epidemiological studies have been important in elucidating the role of water in many infectious diseases. In particular, epidemiological methods are central to the investigation of outbreaks of waterborne disease. Furthermore, as will be seen in a subsequent chapter, the results from epidemiological studies are essential in defining several of the input variables for various risk assessment models which are finding important roles in assessing the risks of waterborne disease.