The biological effects of estrogens are thought to be mediated by two receptors referred to as ERalpha and ERbeta. In recent years significant efforts have been devoted to the design of subtype selective ligands. These ligands are valuable tools to establish the precise biological role of each of the subtypes and to develop new generations of therapeutics. The first part of this review briefly summarizes the biology behind the estrogen receptors. The second part addresses the structure-activity relationship of the subtype selective ER ligands that were reported up to now. In the third part, the current insights in the therapeutic prospects of the subtype selective estrogens will be discussed.