Eugenics is a set of largely discredited beliefs and practices that aim to improve the genetic quality of a human population. Historically, eugenicists have attempted to alter the frequency of various human phenotypes by inhibiting the fertility of those considered inferior, or promoting that of those considered superior.

The contemporary history of eugenics began in the late 19th century, when a popular eugenics movement emerged in the United Kingdom, and then spread to many countries, including the United States, Canada, Australia, and most European countries (e.g., Sweden and Germany).

Historically, the idea of eugenics has been used to argue for a broad array of practices ranging from prenatal care for mothers deemed genetically desirable to the forced sterilization and murder of those deemed unfit. To population geneticists, the term has included the avoidance of inbreeding without altering allele frequencies; for example, British-Indian scientist J. B. S. Haldane wrote in 1940 that "the motor bus, by breaking up inbred village communities, was a powerful eugenic agent." Debate as to what qualifies as eugenics continues today.

A progressive social movement promoting eugenics had originated in the 19th century, with diverse support, but by the mid 20th century the term was closely associated with scientific racism and authoritarian coercion. With modern medical genetics, genetic testing and counseling have become common, and new or liberal eugenics rejects coercive programs in favor of individual parental choice.