The UK Adult Twin Registry was started in 1993 and consists of approximately 10,000 monozygotic (MZ) and dizygotic (DZ) adult Caucasian twins aged 16 to 85 years from all over the United Kingdom, plus some parents and siblings. It now incorporates previous twin registries from the Institute of Psychiatry and Aberdeen University. This is a volunteer sample recruited by successive media campaigns without selecting for particular diseases or traits. All twins receive a series of detailed disease and environmental questionnaires. The majority of twins have been assessed in detail clinically at several time points for several hundred phenotypes related to common diseases or intermediate traits. The focus to date has been primarily in 5 areas--cardiovascular, metabolic, musculoskeletal, ophthalmologic diseases as well as the aging process. Over 3000 DZ twins have had a 10cM genome-wide scan performed and 5000 twins tagged for over 200 candidate genes allowing both linkage and association studies. The resource has led to many successful and innovative research projects particularly in common age-related diseases, and has led to collaborations with over 80 groups worldwide.