The hormone estradiol has effects on many tissues in both males and females. Some of these effects, such as inhibition of cancer growth and modulation of the devastating effects of aging on bone, brain, skin and bladder, are good. Others, such as the effect on the breast and endometrium, are undesirable. The task of designing drugs that would have only the good effects of estradiol has, until recently, seemed almost impossible because it was thought that there was only one estrogen receptor and that an estrogenic agent was definitively categorized as an estrogen agonist or an antagonist. More recently we have learnt that there are two estrogen receptors which, under certain conditions, have opposite effects on gene transcription. In addition, it is now understood that agents cannot be described as agonists or antagonists because a single agent can be an agonist in one tissue and an antagonist in another. The term 'selective' estrogen receptor modulator' was designed to incorporate this. The idea of estrogen receptor modulators has raised new hope that tissue specific estrogens or anti-estrogens can be designed.