 William David Ross (1877–1971) made contributions to ancient philosophy and to moral philosophy. The focus of this entry is his work in the latter area. Ross’s The Right and the Good is one of the most important contributions to moral philosophy published in the twentieth century. In it and other works, Ross develops a novel (pluralistic) deontological ethical theory rivalling Kantianism and utilitarianism. The most innovative element of this theory is the notion of a prima facie duty. Although Ross’s work in moral philosophy appeared to suffer at the hands of critics in the middle and late parts of the last century, recent work on normative and meta-ethical intuitionism has sparked a renewed interest in and enthusiasm for his ethical outlook.