Male fertility has seldom been studied in natural populations because it has been assumed that strong selection would result in uniformly high values among males, and therefore mating success has been equated with fertilisation success. In contrast, male fertility has received much attention in studies of domestic livestock, where economic benefits rely on improving productivity, and in human infertility studies, where the efficiency of treatments depends on understanding which ejaculate traits explain reproductive failures and predict success at assisted conception. Despite years of efforts, no conclusive results have been obtained, probably because such studies have focused on opposite extremes of the range with little variation: domestic livestock have often been subject to strong artificial selection for high fertility, and human patients requiring treatment have compromised fertility. Recent findings from natural populations of red deer have shown that males differ markedly in their fertility, and have revealed the degree of variation found in different semen traits, both between and within males. Fertility trials have shown that male fertility is determined mainly by sperm swimming speed and the proportion of normal sperm, when sperm numbers are kept constant. Sperm design exerts a strong influence on sperm swimming speed, with faster swimming sperm having elongated heads, shorter midpieces and a longer principal plus terminal pieces in relation to total flagellum length. Thus, the large inter-male variation in sperm design found among natural populations underlies differences in sperm swimming speed which, in turn, determine differences in male fertility rates. Secondary sexual characters are honest indicators of male fertility, so males with large and elaborated antlers have larger testes and faster swimming sperm. Testosterone does not seem to mediate the relationship between antler size and semen quality, since it is associated with sperm production, but not with sperm quality or antler size. Finally, more fertile males produce a greater proportion of sons, who will inherit the semen traits which will enhance their fertility.