Phytoestrogens are trace substances in our food which mimic and supplement the action of the body's own hormone, estrogen (sometimes spelt as 'oestrogen'). They are a comparatively recent discovery, and researchers are still exploring the nutritional role of these substances in such diverse metabolic functions as the regulation of cholesterol, and the maintaining of proper bone density post menopause.

Phytoestrogens mainly fall into the class of flavonoids: the coumestans, prenylated flavonoids and isoflavones are three of the most potent in this class. Lignan has also been identified as a phytoestrogen, although it is not a flavonoid. The estrogenic properties of these biochemicals have been shown to be due to their structural similarities to the hormone estradiol.