The Nordic countries have developed similar advanced register data systems for purposes of producing official statistics and research. The register's advantages in criminological research include the ability to maintain data on the total population; the possibility of studying small subpopulations; a virtually continuous timeline in longitudinal dataset; using panel data designs with no sample attrition; having few or no non-responses or other missing data; making connections between different observation units, such as family members; and the ability to construct research designs that are practically impossible with surveys. Use of register data for research requires legislation that protects the privacy of individuals and regulates access to data. Register data have been used extensively in demographic, economic, and sociological research. There is a huge and largely untapped potential for their use in criminological research. (Published Abstract)