Neuroepidemiology is concerned with the study of incidents of neurological disease occurring in a community. It deals with the distribution and dynamics of neurological disease in human populations and the factors that affect these patterns. To estimate the magnitude of the disease burden in the population, the neuroepidemiologist calculates indices of disease, such as mortality, prevalence, and incidence. In studying factors associated with disease, the investigator must establish whether the relationship (1) is artifactual, (2) appears to be the result of a chance occurrence or coincidence, and (3) has any biologic significance. The clinical neurologist is the central figure in neuroepidemiologic investigations. The greater the accuracy and completeness of the neurologist's information, the better the quality of neuroepidemiologic studies based on these data.