Epidemiology is the study of populations using defined research methods to confirm the patterns and causes of disease and applying this information to improve the health of the populations. This branch of science is the basis for understanding the spread of diseases in a defined area or group of people. Epidemiologic studies have been instrumental in improving outcomes by establishing preventive and therapeutic measures for the incidence, prevalence and mortality from cancers. The incidence of malignancy in a country or in our world defines the magnitude of the cancer problem. This chapter in Cancer Concepts: A Guidebook for the Non-Oncologist will introduce the role of epidemiology in the diagnosis and treatment of cancer.https://escholarship.umassmed.edu/cancer_concepts/1002/thumbnail.jp