Social discounting aims to compare the respective future consequences of differing courses of action for human well-being, and so to help decide on policies for matters as varied as climate change, transport and criminal justice. Social discounting is widely used, though some decisions are too trivial, or too urgent, for it to be justified. Even so, its pervasive use is at variance with scepticism about its moral foundations, and about whether the comparisons that it claims to make can be made at all. Debate has, however, concentrated on how, rather than on whether, social discounting should be done and the conditions upon which it must be based seem never to have been set out systematically. This thesis aims to fill that gap, by explaining the moral and practical conditions that must be met for social discounting to be justified. The conditions are demanding. It behoves policy makers to satisfy themselves more carefully than is now done that the conditions are met in respect of the decisions where use of social discounting is proposed, and to consider alternatives where one or more conditions is not met. The thesis takes for granted that human well-being counts morally. But social discounting requires that well-being is capable of being described through an objective list of desiderata and that some aspect of well-being is measurable, at least on a cardinal scale and inter-personally, implying commensurabilities amongst some of the things comprising or contributing to well-being. Some moral theories incorporate priorities, such as property rights or the interests of poor people. Priorities range from easy to meet to very difficult. Priorities of the latter type are inconsistent with social discounting, and are the basis for theories as varied as those of Nozick and Nussbaum. This thesis suggests that the theories consistent with social discounting may collectively be called 'moderate welfarism'. Moderate welfarism allows room for priorities and other moral considerations provided only that the monetisable aspect of social well-being is morally important. Moderate welfarism is necessary but not sufficient for social discounting to be justifiable. Practical difficulties may make it incapable of implementation. One such difficulty is the well-known epistemic problem, but the thesis sets out nine such difficulties, each implying a condition that must be met if social discounting is to be capable of practical use. The thesis concludes that the moral and practical conditions that must be met for social discounting to be justified are demanding and, more speculatively, that some of the conditions are not widely understood leading to inappropriate use of the technique by governments.