Other than its well-recognized effects on reproductive physiology, estrogen has important actions in a wide variety of other body systems with important examples including bone, blood vessels and the heart. These effects are seen in both females and males. Investigators have hypothesized that genetic variants in the genes coding for estrogen signalling proteins may cause variable sensitivity to the hormone and influence an individuals estrogen-sensitive phenotypes. The most obvious candidate genes are the estrogen receptors alpha and beta (ERα and β). However, the regulation of these genes is complex and not well understood. Furthermore, their coding exons, and regulatory sequences are dispersed across large segments of the genome. A number of common polymorphisms have been identified in both ERα and ERβ, with variable degrees of evidence of their direct biological significance and their association with human disease. The identification of genetic variations associated with altered estrogen response is of potential public health importance. Insights may be gained into the pathogenesis of estrogen sensitive diseases such as osteoporosis, breast cancer and cardiovascular disease contributing to the development and application of newer therapies for these disorders. Furthermore, genetic variants that alter sensitivity to estrogen may affect both therapeutic and harmful responses to exogenous estrogen administered in the form of the oral contraceptive pill or hormone replacement therapy. This clinical significance has led to the publication of a number of patents which will be reviewed.