Data related to use of electronic communications services play an important role in the investigation of crime and protection of national security. To secure the availability of such data, legislation has been passed imposing obligations of data retention on service providers. Data retention interferes with rights to privacy and data protection, and to be lawful in relation to these rights the legal regulation must fulfil several conditions. Uncertainty about the exact limitations and conditions applicable to the regulation, has led to disparate development of the legal regulation of data retention in the Nordic countries. The report compares the data retention regulation in the Nordic countries in force as per November 2023 and analyses the differences. Important findings are that the greatest problems concern data retention related to online services, and that legal concepts that prima facie are similar, still may have different meanings in different countries.