Eugenics is a scientific field involving the controlled breeding of humans in order to achieve desirable traits in future generations. Eugenics was at its height in the early decades of the 20th century and was largely abandoned with the end of World War II. At its zenith, the movement often pursued pseudoscientific notions of racial supremacy and purity.

Eugenics was not confined to any one country or culture, but was practiced around the world and was promoted by governments, and influential individuals and institutions. Its advocates regarded it as a social philosophy for the improvement of human hereditary traits through the promotion of higher reproduction of certain people and traits, and the reduction of reproduction of certain people and traits. Today it is widely regarded as a brutal movement which inflicted massive human rights violations on millions of people. The "interventions" advocated and practised by eugenicists involved prominently the identification and classification of individuals and their families, including the poor, mentally ill, blind, 'promiscuous women', homosexuals and entire "racial" groups——such as the Roma and Jews——as "degenerate" or "unfit"; the segregation or institutionalisation of such individuals and groups, their sterilization, their "euthanasia", and in the worst case of Nazi Germany, their mass extermination. The practices engaged in by eugenicists involving violations of privacy, attacks on reputation, violations of the right to life, to found a family, to discrimination are all today classified as violations of human rights. The practice of negative racial aspects of eugenics, after World War II, fell within the definition of the new international crime of genocide, set out in the Convention on the Prevention and Punishment of the Crime of Genocide.

The modern field and term were first formulated by Sir Francis Galton in 1883, drawing on the recent work of his cousin Charles Darwin. From its inception eugenics was supported by prominent people, including H. G. Wells, Woodrow Wilson, Theodore Roosevelt, Emile Zola, George Bernard Shaw, John Maynard Keynes, William Keith Kellogg, Margaret Sanger, Winston Churchill, and Sidney Webb. Its most infamous proponent and practitioner was however Adolf Hitler who praised and incorporated Eugenic ideas in Mein Kampf, and emulated Eugenic legislation for the sterilization of "defectives" that had been pioneered in the United States. G. K. Chesterton was an early critic of the philosophy of eugenics, expressing this opinion in his book, Eugenics and Other Evils. Eugenics became an academic discipline at many colleges and universities, and received funding from many sources. Three International Eugenics Conferences presented a global venue for eugenicists with meetings in 1912 in London, and in 1921 and 1932 in New York. Eugenic policies were first implemented in the early 1900s in the United States. Later, in the 1920s and 30s, the eugenic policy of sterilizing certain mental patients was implemented in a variety of other countries, including Belgium, Brazil, Canada, and Sweden, among others. The scientific reputation of eugenics started to decline in the 1930s, a time when Ernst RĂ¼din used eugenics as a justification for the racial policies of Nazi Germany, and when proponents of eugenics among scientists and thinkers prompted a backlash in the public. Nevertheless, the second largest known eugenics program, created by social democrats in Sweden, continued until 1975.

Since the postwar period, both the public and the scientific communities have associated eugenics with Nazi abuses, such as enforced racial hygiene, human experimentation, and the extermination of undesired population groups. However, developments in genetic, genomic, and reproductive technologies at the end of the 20th century have raised many new questions and concerns about what exactly constitutes the meaning of eugenics and what its ethical and moral status is in the modern era.