Eugenics ( yoo-JEN-iks; from Ancient Greek εύ̃ (eû) 'good, well', and -γενής (genḗs) 'come into being, growing') is a fringe set of beliefs and practices that aim to improve the genetic quality of a human population. Historically, eugenicists have attempted to alter human gene pools by excluding people and groups judged to be inferior or promoting those judged to be superior. In recent years, the term has seen a revival in bioethical discussions on the usage of new technologies such as CRISPR and genetic screening, with a heated debate on whether these technologies should be called eugenics or not.

The concept predates the term; Plato suggested applying the principles of selective breeding to humans around 400 BC. Early advocates of eugenics in the 19th century regarded it as a way of improving groups of people. In contemporary usage, the term eugenics is closely associated with scientific racism. Modern bioethicists who advocate new eugenics characterize it as a way of enhancing individual traits, regardless of group membership.

While eugenic principles have been practiced as early as ancient Greece, 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. In this period, people from across the political spectrum espoused eugenic ideas. Consequently, many countries adopted eugenic policies, intended to improve the quality of their populations' genetic stock. Such 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. Those deemed "unfit to reproduce" often included people with mental or physical disabilities, people who scored in the low ranges on different IQ tests, criminals and "deviants", and members of disfavored minority groups.

The eugenics movement became associated with Nazi Germany and the Holocaust when the defense of many of the defendants at the Nuremberg trials of 1945 to 1946 attempted to justify their human-rights abuses by claiming there was little difference between the Nazi eugenics programs and the U.S. eugenics programs. In the decades following World War II, with more emphasis on human rights, many countries began to abandon eugenics policies, although some Western countries (the United States, Canada, and Sweden among them) continued to carry out forced sterilizations. Since the 1980s and 1990s, with new assisted reproductive technology procedures available, such as gestational surrogacy (available since 1985), preimplantation genetic diagnosis (available since 1989), and cytoplasmic transfer (first performed in 1996), concern has grown about the possible revival of a more potent form of eugenics after decades of promoting human rights.

A criticism of eugenics policies is that, regardless of whether negative or positive policies are used, they are susceptible to abuse because the genetic selection criteria are determined by whichever group has political power at the time. Furthermore, many criticize negative eugenics in particular as a violation of basic human rights, seen since 1968's Proclamation of Tehran, as including the right to reproduce. Another criticism is that eugenics policies eventually lead to a loss of genetic diversity, thereby resulting in inbreeding depression due to a loss of genetic variation. Yet another criticism of contemporary eugenics policies is that they propose to permanently and artificially disrupt millions of years of human evolution, and that attempting to create genetic lines "clean" of "disorders" can have far-reaching ancillary downstream effects in the genetic ecology, including negative effects on immunity and on species resilience.