The Danish Civil Registration System (CRS) is a comprehensive administrative register established on April 2, 1968, that contains individual-level information on all residents of Denmark and Greenland. By January 2014, the CRS had registered 9.5 million individuals and over 400 million person-years of follow-up. A unique ten-digit Civil Personal Register (CPR) number is assigned to each person, enabling easy and cost-effective record linkage across Danish registers. The CRS facilitates nationwide cohort studies with long-term follow-up on emigration and death, and is useful for sampling comparison groups in various study designs. The data are virtually complete, highly accurate, and can be accessed for research while protecting the anonymity of Danish residents. The CRS is a key tool for epidemiological research in Denmark, enhancing the potential of nationwide registers for epidemiological studies.The Danish Civil Registration System (CRS) is a comprehensive administrative register established on April 2, 1968, that contains individual-level information on all residents of Denmark and Greenland. By January 2014, the CRS had registered 9.5 million individuals and over 400 million person-years of follow-up. A unique ten-digit Civil Personal Register (CPR) number is assigned to each person, enabling easy and cost-effective record linkage across Danish registers. The CRS facilitates nationwide cohort studies with long-term follow-up on emigration and death, and is useful for sampling comparison groups in various study designs. The data are virtually complete, highly accurate, and can be accessed for research while protecting the anonymity of Danish residents. The CRS is a key tool for epidemiological research in Denmark, enhancing the potential of nationwide registers for epidemiological studies.