Mary Anne Warren explores a theoretical question which lies at the heart of practical ethics: what are the criteria for having moral status? In other words, what are the criteria for being an entity towards which people have moral obligations? Some philosophers maintain that there is one intrinsic property--for instance, life, sentience, humanity, or moral agency. Others believe that relational properties, such as belonging to a human community, are more important. In Part I of the book, Warren argues that no single property can serve as the sole criterion for moral status; instead, life, sentience, moral agency, and social and biotic relationships are all relevant, each in a different way. She presents seven basic principles, each focusing on a property that can, in combination with others, legitimately affect an agent's moral obligations towards entities of a given type. In Part II, these principles are applied in an examination of three controversial ethical issues: voluntary euthanasia, abortion, and the moral status of animals.