Appeals to human nature as a normative principle for practical reasoning are often made in medical and health-care ethics. Such moral arguments presuppose an essentialist theory of human nature that posits the existence of an intrinsic human nature or essence that explains the necessary properties and causal powers that distinguish human beings from other animals. With an essentialist theory in hand, moral philosophers who advocate a metaethical theory called ethical naturalism propose that moral principles to guide human action, especially the identification of the virtues, can be discerned from and justified by considerations of human nature. Three areas in medicine and health care where human nature has been proposed as a normative principle to guide moral decision-making include attempts to describe the virtuous patient and health-care provider, debates over the legitimacy of biotechnological enhancements of human capacities, and arguments against the moral acceptability of reproductive cloning and other efforts to radically change human procreation.