For 150 years, Down's Syndrome has constituted the archetypal mental disability, easily recognisable by distinct facial anomalies and physical stigmata. In a narrow medical sense, Down's syndrome is a common disorder caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British asylum medical superintendent who described the syndrome as Mongolism in a series of lectures in 1866. In 1959, the disorder was identified as a chromosome 21 trisomy by the French paediatrician and geneticist Jerome Lejeune and has since been known as Down's Syndrome (in the English-speaking world) or Trisomy 21 (in many European countries). But children and adults born with this chromosomal abnormality have an important collective history beyond their evident importance to the history of medical science. David Wright, a Professor of History at the Institute for Health and Social Policy, McGill University, looks at the changing social responses to Down Syndrome from Medieval Europe to the present day in the first ever history of Down Syndrome.