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.

Eugenic programs included both positive measures, such as encouraging individuals deemed particularly "fit" to reproduce, and negative measures, such as marriage prohibitions and forced sterilization of people deemed unfit for reproduction.

Positive eugenics is aimed at encouraging reproduction among the genetically advantaged, for example, the intelligent, the healthy, and the successful. Possible approaches include financial and political stimuli, targeted demographic analyses, in vitro fertilization, egg transplants, and cloning. Negative eugenics aimed to eliminate, through sterilization or segregation, those deemed physically, mentally, or morally undesirable. This includes abortions, sterilization, and other methods of family planning. Both positive and negative eugenics can be coercive; in Nazi Germany, for example, abortion was illegal for women deemed by the state to be superior.

Euthenics (/juːˈθɛnɪks/) is the study of improvement of human functioning and well-being by improvement of living conditions. "Improvement" is conducted by altering external factors such as education and the controllable environments, including environmentalism, education regarding employment, home economics, sanitation, and housing, as well as the prevention and removal of contagious disease and parasites.

In a New York Times article of May 23, 1926, Rose Field notes of the description, "the simplest [is] efficient living". It is also described as "a right to environment", commonly as dual to a "right of birth" that correspondingly falls under the purview of eugenics.