Isoflavonoids with estrogenic activities were discovered in the 1930s for their adverse effects on Sheep reproduction. Then, toxicological studies were performed until the 1980s and the reprotoxic effects of isoflavones (Isofl) were confirmed. However, when these compounds were found in edible plants and in human fluids many questions surged. Isofl being mainly present in soybean, the Asian populations were scrutinized. In 2008, the U.S. National Toxicology Program showed the reprotoxic and carcinotoxic effects of the main soybean Isofl at edible doses. Thus, nowadays, after considering only the beneficial effects of Isofl in humans, researches try to study their potential deleterious effects on reproductive and thyroid functions. Isofl mimic the ubiquitous effects of estradiol and thyroid hormones in mammals. Nearly all biological functions are influenced by xenoestrogens which include drugs and endocrine disruptors. Their effects depend on their concentrations in body fluids and their affinity for hormone receptors. Additionally, gut flora can influence the nature and effects of the Isofl metabolites which reach the target cells. Therefore, in vitro data should be considered cautiously when they involve pharmacological doses of crude extracts or pure chemicals. After an introduction, estrogenic mechanisms are explained and studied for the main estrogenic Isofl. Then are presented plant sources of Isofl, their forms, and their roles in plants. Next are exposed dosages in food and bioavailable forms in humans before considering the beneficial and adverse effects of estrogenic Isofl in humans. New perspectives about food processing impact on Isofl levels in foodstuffs are finally presented.