This collection of essays is dedicated to the memory of the late Harvard philosopher Robert Nozick, who died in 2002. The publication of Nozick's Anarchy, State, and Utopia in 1974 revived serious interest in natural rights liberalism, which, beginning in the latter half of the eighteenth century, had been eclipsed by a succession of antithetical political theories including utilitarianism, progressivism, and various egalitarian and collectivist ideologies. Some of our contributors critique Nozick's political philosophy. Other contributors examine earlier figures in the liberal tradition, most notably John Locke, whose Second Treatise of Government, published in the late seventeenth century, profoundly influenced the American founders. The remaining authors analyze natural rights liberalism's central doctrines.