Epidemiology is the study of the occurrence, distribution, causes, and effects of health-related events and disorders or disease states in human populations. Results from epidemiological studies help identify patterns and risk factors for diseases; these insights then inform the targeting, planning, and evaluation of preventive health care strategies. Epidemiological findings can translate to decisions in health policy to reduce the burden of disease in the population. This entry describes four core epidemiological concepts: measures of disease frequency and association, effect measure modification, commonly employed study designs, and biases that may arise in epidemiological studies.