Phytoestrogens are defined as naturally occurring plant compounds that are structurally and functionally similar to 17 beta-estradiol or that produce estrogenic effects. The commonest sources are cereals, legumes and grasses. Isoflavones are the most highly investigated subgroup of phytoestrogens. They are attenuated estrogens and behave both in vivo and in vitro as agonists and antagonists. The highest concentrations are found in soy beans and legumes. The relative potencies of isoflavones as compared to estradiol are small but they can exhibit bioactivity when tested in high concentrations. A high dietary intake of phytoestrogens was first noted to be associated with a decreased incidence of certain diseases. This epidemiological information was obtained primarily from studying Asian populations. Soy consumption is highest in Japan, where urinary levels of phytoestrogen metabolites are extremely high, and where there are lower rates of so-called 'Western' diseases, namely breast, endometrial, colon and prostatic cancers as well as atherosclerotic disease. These observations have prompted extensive research, which has demonstrated the varying degrees of estrogenicity of these phytoestrogen compounds. This article provides an epidemiological background to phytoestrogens, a brief description of their composition and biochemistry, and an overview of the literature to date on phytoestrogens with an emphasis on relief of menopausal symptoms.