Phytoestrogens (PE) are nonsteroidal compounds of plant origin that can bind to estrogen receptors (ER) as agonists and consequently mimic biological actions of endogeneous estrogens. The majority of plant metabolites that exhibit affinity for ER belong to a definite structural category known as phenylpropanoids (phytochemicals combining aromatic C6 and aliphatic C3 fragments), divided into classes of flavonoids and lignans. These compounds, phenolic in character, represent a very old evolutionary trait and function in plants as optical signals for pollinating insects, stimulators of nodulation, phytoalexins, radiation screens, herbivore deterrents, etc., while estrogenic activity does not appear to grant to the host any particular environmental advantage. Conversely, animals that feed on plants containing such constituents can suffer from an acquired hormonal imbalance, including infertility.