We describe the Catholic natural law tradition by examining its origins in the medieval penitentials, the papal decretals, the writings of Thomas Aquinas, and seventeenth century casuistry. Catholic natural law emerges as a flexible ethic that conceives of human nature as rational and as oriented to certain basic goods that ought to be pursued and whose pursuit is made possible by the virtues. We then identify four approaches to natural law that have evolved within the United States during the twentieth century, including the traditionalist, proportionalist, right reason, and historicist approaches. The normative implications of these approaches are discussed in relation to ethical issues in the tobacco industry, ITT under Geneen, the marketing of pharmaceuticals, affirmative action, and bribery. It is argued that Alasdair MacIntyre is correct in claiming that the natural law tradition is superior to the liberal ethics of modern deontology and utilitarianism.