This paper explores how uncertainty over investment returns affects pension systems. This issue is becoming more important because of the dramatic spread of defined-contribution pension provision around the world. It has also been highlighted by the recent financial crisis: the OECD estimates that pension funds lost 23% of their value in 2008, worth a heady USD 5.4 trillion. The scale of investment risk is measured in this paper using historical data on returns on equities and bonds in major OECD economies over the past quarter century. The results show a median real return of 7.3% a year on a portfolio equally weighted between equities and bonds (averaging across the countries studied). It might be expected that, over a very long period, the degree of uncertainty in investment returns is small. After all, a few bad years in the market are likely to be offset by boom years. Nevertheless, the degree of uncertainty, even with the relatively long investment horizons of pensions, is found to be large. In 10% of cases, an annual return of less than 5.5% would be expected, while in 10% of cases, this should exceed 9.0%. Compounded over the time horizon for pension savings of 40 years or more, such differences in rates of return amount to enormous sums of money. However, there is a series of reasons why returns achieved by individuals on their pension funds are less than the market return (as measured by conventional indices). These factors include administrative charges, agency and governance effects and demographic change, depressing investment returns below the high levels recorded over the past two decades. As a result, a more conservative assumption for future investment returns than the record over the past quarter century is appropriate. Settling on a median of 5.0% annual real return net of charges implies that 80% of the time, the investment return on pension savings should be between 3.2% and 6.7% a year.