It is common for fetal" viability" to be dismissed out of hand as a morally arbitrary or problem-ridden criterion for fetal" personhood." In this essay I want to examine and reject one particular reason often advanced in support of this claim, namely that future medicaltechnological progress is almost certain to someday render a fetus viable at the earliest stages of pregnancy, perhaps even at conception. Roger Wertheimer writes that" the viability of a fetus is its capacity to survive outside the mother, and that is totally relative to the state of available medical technology. In principle, eventually the fetus may be deliverable at any time, perhaps even at conception. The prob-lems this poses for liberals are obvious...." i But what exactly are these" obvious problems"? I shall argue that no obvious problems, perhaps no problems whatsoever, follow from the mere fact that someday even a fertilized ovum may be viable. First we must clearly understand what is meant by" viability" and why some, including the United States Supreme Court, 2 have thought i. Roger Wertheimer," Understanding the Abortion Argument," Philosophy & Public Affairs I, no. i (Fall I97I), abridged and reprinted in Joel Feinberg, ed., The Problem of Abortion (Belmont, CA: Wadsworth Publishing Co., I973), p. 43. Since all the references below are found in the Feinberg collection, I shall use the pagination in Feinberg throughout. Wertheimer's objection is echoed by others; see, for example, John Noonan, Jr.," An Almost Absolute Value in History," Problem of Abortion, p. ii, excerpted from John T. Noonan, Jr., The Morality of Abortion: Legal and Historical Perspectives (Cambridge, MA: Harvard University Press, I970).