The Norwegian Twin Registry (NTR) is a large population based twin cohort for research purposes. At present,the registry has 14 692 complete twin pairs with information on zygosity and to varying degree information onsomatic and mental health, lifestyle and demographics. The registry covers birth years 1895-1960 and 1967-1991. NTR was established in 2009, at the Norwegian Institute of Public Health, as a merger of three majortwin panels, the oldest originating in the 1960s. Since then Norwegian twin research has been a notablecontributor to twin research internationally. Norwegian twin researchers have published over 250 papers basedon Norwegian twin data, spanning a broad range of somatic and mental health phenotypes. In twin studies ofheritability a data structure with both variance within and between pairs is required. Therefore a large sample isnecessary, especially when studying rare diseases and conditions, and it is of vital importance to expand theregistry. NTR is actively recruiting new twins, both young and older, but declining response rates are achallenge. The value of NTR is greatly enhanced through the linkage possibilities offered by Norway’s manynationwide registries (medical, demographic, and socio-economic). Access to data is permitted by the NTRsteering group and will in most instances need permission from the Regional Ethics Committee.