From the earliest calls to kill people with disabilities—either with or without their consent—in the mid-nineteenth century, until today, euthanasia proponents have called upon science to justify their program. The first German intellectual to propose infanticide for children born with disabilities was the biologist Ernst Haeckel in 1870. He based his argument on the new scientific understanding of humanity emerging in the wake of the Darwinian revolution. Similar ideas featured prominently in the first public discussion over euthanasia in Britain, which also began in the 1870s. Public debate over euthanasia did not begin in the United States until the 1890s, but scientific justifications for euthanasia were a prominent part of the discussion. Despite many philosophers’ insistence that such reasoning violated the naturalistic fallacy by crossing the is-ought divide, many scientists ignored these warnings and continued promoting euthanasia, especially as the eugenics movement gained momentum in the early twentieth century. By the 1920s-30s many German scientists and medical personnel regarded euthanasia as a logical corollary of their allegedly scientific worldviews. Thus the Nazi regime found many willing accomplices in the medical profession when they began their program to exterminate people with disabilities, which resulted in the murder of over 200,000 people. The backlash against the Nazi atrocities brought euthanasia into disrepute in the late twentieth century. Nonetheless, some euthanasia proponents continue to insist that science—especially Darwinian science—supports their perspective. The bioethicists Peter Singer and James Rachels, for instance, use arguments very similar to those advanced already in the late nineteenth century. In addition to presenting this history as a cautionary tale, in this paper I intend to examine and critique the philosophical problems with the euthanasia proponents’ use of “science” to promote their ideology.