Chemically, isoflavones belong to the group of polyphenols. The most important food source is soy, which contains mainly genistein and daidzein in the form of glycosides. The absorption rate ranges from 20-55%. Isoflavones are selectively incorporated in certain tissues like the breast and ovaries. They are able to bind to the estrogen receptors alpha (ER-alpha) and beta (ER-beta). However, the binding affinity for genistein to ER-alpha is only 4%, the affinity to ER-beta is 87% compared to 17beta-estradiol. Thus, depending on the estradiol concentration, they exhibit weak estrogenic or antiestrogenic activity. Isoflavones can influence transcription and cell proliferation. They modulate enzyme activities as well as signal transduction, and have antioxidant properties. Epidemiological studies have shown that the prevalence of hot flashes is lower in women from countries with high dietary isoflavone intake such as Japan than in Western nations with low isoflavone intake. Results of clinical studies on the effects of soy products or isolated isoflavones on vasomotor symptoms are contradictory. Due to a strong placebo effect and a time-dependent reduction of hot flashes, phytoestrogens were seen to have no significant effect in most studies. The use of soy isoflavones could, however, be considered for women with intense disorders. Because of contradictory data from in vitro and animal studies, a definite safety assessment of isoflavone intake cannot be made to date. While moderate isoflavone consumption seems to be safe in the majority of the population, women with breast cancer should avoid long-term use of soy products or isolated isoflavones.