Following publications underlining the potential use of selenium (Se) in reducing the risk of prostate, skin, colorectal, liver, mammary and lung cancer, awareness of the importance of Se to human health has markedly increased. Moreover, Se status has been inversely associated with other health problems, such as impaired immune function, arthropathy and cardiomyopathy. The most important and well-known Se functions are represented by cell protection against oxidative stress and regulation of thyroid hormone metabolism. Recently, ongoing studies have focused on the relationship between Se intake and fertility and reproductive pathology, as demonstrated by the finding of low Se levels in blood and placenta of women suffering pre-eclampsia and intrauterine growth retardation. Similar problems have also been investigated in livestock and, since the concentration of Se in different soils and different geographical regions varies, addition of this element to animal feed is often required to prevent Se-deficiency diseases in production animal systems. Furthermore, Se supplementation in cattle and ewes is associated with increased embryo production, higher fetal mass and reduced incidence of retained placenta. However, the beneficial effects of supranutrional diet – above European Commission and Food and Drug Administration recommendations (0.5 and 0.3 mg kg–1 dm, respectively), but below the maximum tolerable level established by the National Research Council (5 mg kg–1 dm) – are affected by other environmental, nutritional and management factors (source of Se, time and length of the treatment, presence of interfering elements, diet feeding pattern). The relationship between Se supplementation and the risk of reproductive diseases are complex but recent developments represent promising results in order to refine dietary recommendations for both humans and livestock and develop effective health strategies.