Dehydroepiandrosterone (DHEA) and DHEA-sulfate (DHEA-S) are the major steroids produced by the adrenal glands. Although high concentrations of these steroids are found in the blood, they were not generally considered to have significant physiological or pathological roles. However, there is now considerable interest among scientists and the general media. This text concerns the role that DHEA/DHEA-S may have in the development of cardiovascular disease, diabetes, some forms of cancer, and perhaps most importantly, in regulating the immune system. DHEA/DHEA-S concentrations in the blood peak between the ages of 20-30 and then decline rapidly so that, by the age of 60, levels are only one third of those in young adults. The possibility of using DHEA/DHEA-S as a new type of "anti-ageing" hormone replacement therapy, to restore levels to those of young adults, is therefore attracting interest.