Abstract Phytoestrogens, also called “dietary estrogens,” are nonsteroidal phytochemicals structurally similar to female hormone estradiol (17-β-estradiol). According to the chemical structure, this group of compounds consists of the following subgroups: isoflavones, coumestans, lignans, as well as stilbenes and prenylated flavonoids. Isoflavones are the most investigated class of phytoestrogens. Soybeans (Glycine max) and red clover (Trifolium pratense) are the most important plant sources of these bioactive nonnutrients. The major isoflavone aglycones in soybeans are genistein, daidzein, and glycitein, which are also present as corresponding glycosides (β-glycoside, acetyl-β-glycoside, malonyl-β-glycoside). Major isoflavones present in red clover include biochanin A and formononetin, while genistein and daidzein are also present. It has been shown that different factors influence the content of isoflavones in plant sources. Various studies are focused on determination of these factors. The aim is to produce and/or select a raw material for further processing into food ingredients or dietary supplements which, due to their favorable characteristics, may enhance health benefits of these products. As bioactive compounds structurally similar to estrogen, phytoestrogens have the ability to bind to estrogen receptors either activating or blocking them, depending on the type of estrogen receptors. Their estrogenic and/or antiestrogenic effects are also shown to be tissue specific. Beside the estrogenic and antiestrogenic activity, phytoestrogens possess capacity for other biological activities. Numerous potential health benefits of these compounds have been investigated, including effects on cancer, vascular diseases, osteoporosis, and menopausal symptoms. Epidemiologic evidence and collected experimental data from animal studies highly suggest beneficial effects of isoflavones on health, but the clinical data in favor of such effects are still to be obtained. Due to their assumed beneficial effects, use of products and supplements containing isoflavones has significantly increased. These products are widely commercialized in many countries as alternative therapy for alleviating menopausal discomforts and for the prevention of diseases such as osteoporosis or breast cancer. Dietary supplements, containing phytoestrogens, are often a blend of extracts from different plants, and usually contain extracts from soy and red clover. Results obtained from analysis of phytoestrogens content in dietary supplements showed no uniformity in the profile and concentration of these compounds among the products of a particular category. The phytoestrogen levels also vary from batch to batch and product to product, causing more difficulties in interpretation of results. Current results indicate that the beneficial effects of phytoestrogens are strongly supported by synergistic effect with other phytochemicals in plants.