Phytoestrogens are plant-derived xenoestrogens (see estrogen) not generated within the endocrine system but consumed by eating phytoestrogenic plants. Also called “dietary estrogens”, they are a diverse group of naturally occurring nonsteroidal plant compounds that, because of their structural similarity with estradiol (17-β-estradiol), have the ability to cause estrogenic or/and antiestrogenic effects, by sitting in and blocking receptor sites against estrogen. Their name comes from the Greek phyto (“plant”) and estrogen, the hormone which gives fertility to female mammals. The word “estrus” – Greek οίστρος – means “sexual desire”, and “gene” – Greek γόνο – is “to generate”. It has been proposed that plants use phytoestrogens as part of their natural defence against the overpopulation of herbivore animals by controlling male fertility. The similarities, at molecular level, of estrogens and phytoestrogens allow them to mildly mimic and sometimes act as antagonists of estrogen. Phytoestrogens were first observed in 1926, but it was unknown if they could have any effect in human or animal metabolism. In the 1940s, it was noticed for the first time that red clover (a phytoestrogens-rich plant) pastures had effects on the fecundity of grazing sheep. Researchers are exploring the nutritional role of these substances in the regulation of cholesterol and the maintenance of proper bone density post-menopause. Evidence is accruing that phytoestrogens may have protective action against diverse health disorders, such as prostate, breast, bowel, and other cancers, cardiovascular disease, brain function disorders and osteoporosis, Phytoestrogens cannot be considered nutrients, given that the lack of these in the diet does not produce any characteristic deficiency syndrome nor do they participate in any essential biological function. Analytical methods are available to determine phytoestrogen content in plants and food.