Epidemiologists investigate the magnitude, distribution, and determinants of diseases in populations. Understanding how the frequency and expression of a disease varies over time, place, and person may yield important clues to its cause. Additionally, public health planners depend on accurate epidemiologic data to develop effective prevention and treatment strategies. This article introduces some basic epidemiologic terms and concepts and reviews the epidemiology of Tourette syndrome (TS). Particular attention will be paid to the unique difficulties associated with studying this disorder. Related topics such as detailed disease phenomenology and genetic epidemiology are discussed elsewhere in this issue.