This book is the first systematic examination of the international eugenics movement, beginning with its origins in the late Victorian period and continuing through the period of 'crypto-eugenics', as the eugenicists themselves dubbed it, after the Second World War and through the 1990s. It dramatically revises the scholarly understanding of the eugenics movement in Britain, Germany, and the United States, but also moves beyond this axis to consider the impact of eugenic ideas on imperial policy and within the colonial world of the late 19th and early 20th centuries. By exploring financial ties between international organisations and utilizing new archival sources such as the Pitt-Rivers correspondence, the author is able to show how and why the eugenics movement was able to command such high levels of support from both scientific practitioners and government officials in the years before the Second World War. It reveals the full relationship between leading eugenicists in Britain and the United States to Adolf Hitler's government in the mid to late 1930s, a relationship that had previously been the subject of much scholarly speculation. It thus makes major contributions to the understanding of twentieth century international relations, the interwar period, and the history of modern biology more generally.