Danish researchers have a unique possibility for producing high-quality observational studies through the Danish registers. The intention of this review is to provide an introduction to data sources, access and linkage of the Danish registers, primarily from Statistics Denmark, the Danish Health Data Authority and the Danish clinical quality databases. Authorisation to data access is given by each institution separately and requires permission by the Danish Data Protection Agency. Data can be linked by the unique Central Person Registry number. Known limitations and possibilities are discussed.