If killing another human being is morally wrong on at least some occasions (as it clearly is), what precisely makes it wrong on those occasions? I have framed the question thus to indicate that I shall not be considering the view that killing another human being is always and everywhere morally wrong. I take it as read that there are at least some morally justifiable killings (for instance, in self-defence where no other means of disarming one's attacker is available). Once it is clear what is wrong with killing on some occasions it should become possible to explain why it is not wrong on others. My immediate concern is with the killing of another human being, and with applying my answer to cases of voluntary and non-voluntary euthanasia, but light will be shed on whether and, if so, why it is wrong to kill oneself, to kill unborn human beings, and to kill non-human animals especially those whose life has most in common with the life of human beings.