With the rapid advance of bio-genetic technology, it will soon be possible for parents to design children who are born with certain genetic traits. This raises the question whether parents should be allowed to use this technology to engineer their children as they please. In this context it is often thought and argued that liberalism, which has a reputation for being permissive of all kinds of practices, grants parents the right to do so. However, I will argue that, on an understanding of liberalism that is identical to the one used by the defenders of genetic design, liberals should wary of such practices. Liberalism, in its most general form, requires that any time individuals exercise power over others they justify it without relying on any particular conception of what a good life is. When we design children to have certain traits that are only useful for realising some conceptions of the good life, we are implicitly endorsing those conceptions. Hence this practice cannot be justified in neutral terms, and liberals should be sceptical of it. Only when we engineer our children to have traits that are useful for all conceptions of the good life can liberals allow the use of this new technology. Indeed, liberalism holds that this is morally required.