The authors examine the effects of mortality trends on age structure and how fertility and migration affect demograpnic aging. "Computations are done for both France and Italy on population ageing during the last 30 years as well as until the end of the official population projections around 2040. Though the ageing effect of fertility decline is usually emphasized, we can see here that the consequences of mortality changes are at least equally (and sometimes more) important. The case of the Italian projection is particularly fascinating. Even if the very low fertility level of 1.4 children per capita is maintained until the year 2040, more than half the increase in the proportion of the population aged 60 and over (from 21% to 43% for females) would be due to mortality change and slightly less to fertility change." (SUMMARY IN FRE)