Moral dilemmas can arise from uncertainty, including uncertainty of the real values involved. One interesting example of this is that of experimentation on human embryos and foetuses. If human embryos or foetuses (henceforth embryos) have a moral status similar to that of human persons then there will be severe constraints on what may be done to them. If embryos have a moral status similar to that of other small clusters of cells then constraints will be motivated largely by consideration for the persons into whom the embryos may develop. If the truth lies somewhere between these two extremes, the embryo having neither the full moral weight of persons, nor a completely negligible moral weight, then different kinds of constraints will be appropriate. So on the face of it, in order to know what kinds of experiments, if any, we are morally justified in performing on embryos we have to know what the moral weight of the embryo is. But then an impasse threatens. For it seems implausible to many to suppose that we have settled the issue of the moral status of human embryos. It is the purpose of this chapter to show that such moral uncertainty need not make rational moral justification impossible. Section 1 develops a framework, applicable to cases of moral uncertainty, which distinguishes between what is morally right/wrong, and what is morally justified/unjustified. In section 2 this framework is applied to the case of embryo experimentation.