The methodological advances in epidemiology have facilitated the use of the Danish Civil Registration System (CRS) in ways not previously described systematically. We reviewed the CRS and its use as a research tool in epidemiology. We obtained information from the Danish Law on Civil Registration and the Central Office of Civil Registration, and used existing literature to provide illustrative examples of its use. The CRS is an administrative register established on April 2, 1968. It contains individual-level information on all persons residing in Denmark (and Greenland as of May 1, 1972). By January 2014, the CRS had cumulatively registered 9.5 million individuals and more than 400 million person-years of follow-up. A unique ten-digit Civil Personal Register number assigned to all persons in the CRS allows for technically easy, cost-effective, and unambiguous individual-level record linkage of Danish registers. Daily updated information on migration and vital status allows for nationwide cohort studies with virtually complete long-term follow-up on emigration and death. The CRS facilitates sampling of general population comparison cohorts, controls in case-control studies, family cohorts, and target groups in population surveys. The data in the CRS are virtually complete, have high accuracy, and can be retrieved for research purposes while protecting the anonymity of Danish residents. In conclusion, the CRS is a key tool for epidemiological research in Denmark.