The Danish Civil Registration System (CRS) is a key tool in epidemiological research. Established on April 2, 1968, the CRS contains individual-level information on all persons residing in Denmark (and Greenland since May 1, 1972). It has cumulatively registered 9.5 million individuals and over 400 million person-years of follow-up by January 2014. A unique ten-digit Civil Personal Register (CPR) number allows for easy, cost-effective, and unambiguous individual-level record linkage of Danish registers. Daily updated information on migration and vital status enables 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. The CRS was established for two main reasons: a growing need for general personal data such as addresses; and the need for identification of individuals for public administration purposes. The CRS registers all persons who are born alive of a mother already registered in the CRS, have their birth or baptism registered in a Danish electronic church register, or reside legally in Denmark for 3 months or more. While newborns are registered at birth, even if they live only for a short time, stillborn children are not. Persons, including newborns, who are entitled to Danish citizenship but live abroad are not registered in the CRS unless they move to Denmark. Upon registration, each person receives a CPR number. Persons who do not fulfill the above criteria for registration but become members of the Danish Labour Market Supplementary Pension Fund (ATP) or are required to pay tax in Denmark also receive a CPR number. Residents of Greenland have been included in the CRS since May 1, 1972. As of January 2, 2014, almost 9.5 million unique CPR numbers have been assigned to residents of Denmark and Greenland, among whom 5.6 million are currently alive and living in Denmark. The CPR number is a unique ten-digit personal identifier that constitutes an essential part of the personal information stored in the CRS. The format of the CPR number is DDMMYY-SSSS, where DDMMYY is the date of birth and SSSS is a serial number that makes it possible to distinguish between persons born on the same day. Digits 5–7 encode the century and year of birth. The last digit of the serial number encodes the person's sex (odd for males and even for females). Previously, the last digit always functioned as a check digit for the entire CPR number and was used to guard against errors caused by the incorrect transcription of a CPR number when used by public authorities. This control system based on check digits is referred to as modulus 1The Danish Civil Registration System (CRS) is a key tool in epidemiological research. Established on April 2, 1968, the CRS contains individual-level information on all persons residing in Denmark (and Greenland since May 1, 1972). It has cumulatively registered 9.5 million individuals and over 400 million person-years of follow-up by January 2014. A unique ten-digit Civil Personal Register (CPR) number allows for easy, cost-effective, and unambiguous individual-level record linkage of Danish registers. Daily updated information on migration and vital status enables 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. The CRS was established for two main reasons: a growing need for general personal data such as addresses; and the need for identification of individuals for public administration purposes. The CRS registers all persons who are born alive of a mother already registered in the CRS, have their birth or baptism registered in a Danish electronic church register, or reside legally in Denmark for 3 months or more. While newborns are registered at birth, even if they live only for a short time, stillborn children are not. Persons, including newborns, who are entitled to Danish citizenship but live abroad are not registered in the CRS unless they move to Denmark. Upon registration, each person receives a CPR number. Persons who do not fulfill the above criteria for registration but become members of the Danish Labour Market Supplementary Pension Fund (ATP) or are required to pay tax in Denmark also receive a CPR number. Residents of Greenland have been included in the CRS since May 1, 1972. As of January 2, 2014, almost 9.5 million unique CPR numbers have been assigned to residents of Denmark and Greenland, among whom 5.6 million are currently alive and living in Denmark. The CPR number is a unique ten-digit personal identifier that constitutes an essential part of the personal information stored in the CRS. The format of the CPR number is DDMMYY-SSSS, where DDMMYY is the date of birth and SSSS is a serial number that makes it possible to distinguish between persons born on the same day. Digits 5–7 encode the century and year of birth. The last digit of the serial number encodes the person's sex (odd for males and even for females). Previously, the last digit always functioned as a check digit for the entire CPR number and was used to guard against errors caused by the incorrect transcription of a CPR number when used by public authorities. This control system based on check digits is referred to as modulus 1