This book is a comprehensive philosophical study of the ethics of killing in cases in which the metaphysical and moral status of the individual killed is uncertain or controversial. Among the questionable and marginal in this way are human embryos, foetuses, neonates, animals, anencephalic infants, congenitally and cognitively-impaired human beings, and human beings who have become severely demented or irreversibly comatose. In an attempt to understand the question of moral status in such cases, The Ethics of Killing develops and defends many different accounts of personal identity, the nature of death, and the wrongness of killing. McMahan contends that the morality of killing is deeply complex and that the principles that determine the morality of killing in marginal cases are different from those that govern the killing of persons who are self-conscious and rational. Among the central claims of the book is that killing in marginal cases should be evaluated primarily in terms of the impact it would have on the victim at the time rather than on the ontological value of the victim's life as a whole. What primarily matters, in other words, is how killing the victim would affect what this particular victim is concerned with at the time of his or her death. In the second half of the book, the various foundational claims about identity, death, and killing are brought to bear in a systematic fashion to lead to conclusions that are both novel and plausible about such practical issues as abortion, prenatal injury, infanticide, the killing of animals, the significance of brain-death, the termination of life-support in cases of permanent vegetative state, the use of anencephalic infants as sources of organ- transplantation, euthanasia, assisted suicide, and advance directives in cases involving dementia. The range and scale of this groundbreaking book is unprecedented.