Many compounds of great structural diversity possess varying degrees of estrogenic activity (Fig. 1 ). Estrogenic substances may contain a steroidal ring system as do, for instance, the ovarian hormones estradiol3 (E2), estrone (El), or the synthetic estrogens 17α-ethinylestradiol or mestranol. Nonsteroidal phenols with estrogenic activity include the potent synthetic estrogen diethylstilbestrol (DES) and environmental substances produced by plants (equol, coumestrol), or microbes (enterolactone). Some industrial chemicals have estrogenic activity, such as bisphenol A and nonylphenol. In addition to hormonal activity, both natural and synthetic estrogenic compounds containing a phenolic structure possess pro-oxidant and/or antioxidant activities.