Flavonoids possessing a 4′-monohydroxylated B-ring are known to be widely distributed in plants of dietary importance. Isoflavones, such as genistein and daidzein, are found in a number of plants including fava beans, soybeans, lupin, kudzu and psoralea being the primary food sources, and even in coffee and medicinal plants, such as Flemingia vestita. Genistein has been studied for many decades now, due to its ability to bind the estrogen receptors and a large variety of other effects independent of its estrogenic activity. This Chapter will focus on the chemical structure of genistein and its biochemical interactions with estrogen receptors.