This article describes the development of the Norwegian Historical Population Register, which is the first open national register. In the period 1735–1964, 9.7 million people lived in Norway, and for them 37.5 million events (such as birth, death, or migration) have been recorded in sources. We link together as many events as possible for the same persons and families, but only include links that have a high probability of being correct. The linking is performed by automatic methods and crowdsourcing. A national population register is important for migration research. It allows us to reconstruct (stepwise) internal migration in Norway, frequently followed by international migration from Norway, as well as return migration to Norway. Many non-Norwegian sources also specify place of birth by country, and this makes it possible to identify individuals in Norwegian sources.