Phytohormones describe substances of plant origin with a hormonal activity. Epidemiological studies have shown a high health care potential for phytoestrogens. They are plant secondary metabolites with structural analogies to estradiol and with an influence on the estrogenic hormonal pathways. The main chemical classes are isoflavones, lignans, coumestans, prenylflavonoids and stilbene derivatives and occur in food and many health products. Soy and soy isoflavones represent an important source of phytoestrogens. Some human natural estrogens and androgens, or analogues, also occur in plants. Other hormones, such as melatonin, have also been found in plants.