The decline in milk yield after peak lactation in dairy animals has long been a biological conundrum for the mammary biologist, as well as a cause of considerable lost income for the dairy farmer. Recent advances in understanding the control of the mammary cell population now offer new insights on the former, and a potential means of alleviating the latter. The weight of evidence now indicates that a change in mammary cell number, the result of an imbalance between cell proliferation and cell removal, is a principal cause of declining production. Further, it suggests that the persistency of lactation, the rate of decline in milk yield with stage of lactation, is strongly influenced by the rate of cell death by apoptosis in the lactating gland. Mammary apoptosis was first demonstrated during tissue involution after lactation, but has now been detected during lactation, in mammary tissue of lactating mice, goats and cattle. Those factors that determine the rate of cell death by apoptosis are as yet poorly characterized, but include the frequency of milking in lactating goats. Initial evidence suggests that nutrition also is likely to influence cell survival after peak lactation, an important factor being the degree of oxidative stress imposed by feed and the tissue's ability to deal with, and prevent damage by, reactive oxygen species. Comparison of cows in calf or not pregnant during declining lactation also indicates a likely influence of reproductive hormones, with oestradiol and progesterone acting to preserve mammary ductal and alveolar integrity during the dry period, while allowing a degree of apoptosis and cell replacement. In each case, the molecular mechanisms controlling mammary cell survival (or otherwise) are as yet poorly defined. On the other hand, more persistent lactations are likely to benefit animal welfare through fewer calvings and by placing less emphasis on maximal production at peak lactation, and modelling of persistent lactation with longer calving intervals indicates their likely economic benefits. In these circumstances, there is considerable incentive to elucidate the determinants of mammary apoptosis, and the factors controlling the dynamic balance between cell proliferation and cell death in the lactating mammary gland.