TwinsUK is the UK’s largest and the world’s most clinically detailed adult twin registry, aiming to investigate the genetic and environmental basis of a range of complex diseases and conditions. The study was set up in 1992 to investigate the incidence of osteoporosis and other diseases in several hundred monozygotic (identical) twins, and since then has expanded to include over 15,000 identical and non-identical twins from across the UK, with ages between 18 and 100. TwinsUK recently obtained approval to recruit twin children of all ages, and so shortly will be able to study health and disease throughout the entire life course.