Most actuaries would agree that the determination of the rate of interest to be employed in the valuation of a pension fund is one of the most difficult of the problems which have to be faced. It is usually possible, either from the actual experience of the staff in question or from a knowledge of the experience of other staffs which are similar in character, to form a fairly accurate estimate of the rates of mortality likely to be experienced before the pension age is reached and, with perhaps rather less certainty, the rates of withdrawal from service. So far as the rates of mortality after retirement are concerned, there is available a considerable volume of data regarding the experience of annuitants and of the pensioners of a number of large funds, and so long as a reasonable allowance is made for future improvement no great difficulty is likely to be experienced on this account. It is true that the experience of some of the larger pension funds has shown that the mortality of pensioners tends to fluctuate over a period of years, but on the whole such a standard can be adopted as will be likely to be adequate for many years.