Like any form of investment, social investment will generate a rate of return and will represent a good use of public funding only if that rate of return is high enough compared to alternative uses of the resources. As with assessments of the benefits of action to prevent climate change, it is important to identify the main costs and benefits over time and to take into account the cumulative results of longer-term effects. However, because many of the consequences of social investment only materialize in the longer run, the justification for making social investments will be crucially affected by the discount rate chosen. This chapter examines how social investment can best be assessed from an economic standpoint, drawing attention to aspects that may prove contentious.