Since 1960 South Africa has seen a steep fall in fertility levels and currently its total fertility rate is the lowest on the African continent. Given the high prevailing levels of fertility in African countries, a better understanding of the factors behind the fertility transition will be valuable not only for South Africa, but also more widely for other African countries. This paper uses the National Income Dynamics Study data to construct a retrospective panel to investigate reasons for the decline in fertility. The analysis attributes a large share of the observed fertility decline across birth cohorts to improvements in education levels and the lower prevalence of marriage. However, a considerable segment of the transition is ascribed to unobservables. These may include HIV/AIDS, the increased use of contraceptives and changes in both intra-household relationships and the social role of women.