The classical phytoestrogens, so far known, constitute a group of plant-derived compounds which include mainly isoflavones, lignans, coumestanes, stilbenes and the flavonoids quercetin and kaempherol. The discovery of many more novel estrogen-like compounds in the plant kingdom demonstrates that the spectrum of phytoestrogens in nature is expanding. The classical as well as the novel phytoestrogens show a complex mode of action via interaction with the nuclear estrogen receptor isoforms ERalpha and ERbeta, exhibiting either estrogen-agonist or estrogen-antagonist effects. Their final biological activity, assessed by cell culture assay systems, animal studies and clinical trials, depends on multiple factors such as the chemical structure of the phytoestrogen, the kind of tissue and cell type, the intrinsic estrogenic status, the route of administration, the metabolism as well as the time and the level of exposure. They are characterized by high tissue specificity and dose-dependent activity. However, although phytoestrogen intake as food or dietary supplements, in particular soya products and the isoflavones genistein and daidzein, has been associated with "health promoting effects", some data indicate increased disease risk. Evidently, phytoestrogen supplementation should be viewed with caution until further studies satisfactorily delineate the effects of individual phytoestrogens on human health and disease.