Early in the twentieth century, a powerful union of science and social policy emerged in countries across the world. Eugenics was a movement committed to using the principles of heredity and of statistics to encourage healthy and discourage unhealthy reproduction. Throughout the twentieth century, but especially in the earlier decades, eugenics played a significant role in shaping government policy. ‘The world of eugenics’ outlines the links between eugenics and social reform and the differences between positive and negative eugenics. It discusses how Nazism and eugenics became so closely connected; the rise of eugenics in science and culture worldwide; the approach to eugenics by different religions; and finally the forms of resistance to eugenics.