All the five Nordic countries have comprehensive population-based health information systems based on individual-focused health registers. Unique identification numbers makes data linkages technically very easy. Registers on cancers have operated since the 1940s, registers on infectious diseases since the 1950s, hospital discharge registers, cause-of-death registers and birth and malformation registers since the 1960s, and health care quality registers and prescription registers since the 1990s. All countries have a register-based census since the 1990s: information on education, income, socioeconomic position, family background and relatives can be linked to the various registers. The current national and European legislations allow the collection of sensitive health and social welfare data without informed consent, and enable the use of such data in scientific research and in for statistical purposes. The main obstacles for Nordic science are differences in register contents, classifications, definitions and data coverages – the harmonisation process may take long time. Also the process to get permissions to combine data from multiple Nordic countries may be complex and time-consuming. The collection of new data sources, such as primary health care data and electronic patient records opens new possibilities for the Nordic countries.