The viability of the fetus has played a significant part in the framing of abortion legislation, but the use of viability as a criterion represents a compromise which is conceptually unclear and which fails to satisfy either side in the abortion debate. Both conservative and liberal views on abortion regard fetal development as morally irrelevant. For the moderate (or gradualist) view some point in the development must be found which indicates a change in the moral status of the fetus. Since viability changes according to available techniques for neonatal care it cannot be the criterion. Alternative criteria are surveyed and it is concluded that the onset of sensation is a significant transition point beyond which abortion and infanticide cannot be morally distinguished. It is suggested that either legislation should be amended to prohibit abortion after 18 to 20 weeks, or abortion to full term should be permitted and the possibility of legislation for infanticide be envisaged.