Netherlands Twin Register (in Dutch Nederlands Tweelingen Register) (NTR) focuses on the etiology of individual differences in health, fertility, development, growth and behavior. The genetic basis is studied by approaches from genetic epidemiology, and other ‘omics’. Large numbers of twin families are needed for the genetic analysis of human complex traits and the elucidation of etiological pathways. NTR collects longitudinal data from over 200,000 participants, including twins (40%) and their family. Of special interest, also to our participants, is the inheritance of twinning and twinning as a risk factor for later health outcomes.