Phytoestrogens, polyphenolic compounds, are one of the major plant secondary metabolites found abundantly in animal diets. However, these phytochemicals play initial important roles in plant growth, development and maintenance. They also have many biological roles in animal body, affecting their productivity. The richest sources of phytoestrogens in animal diets are legumes, particularly, soybeans and clover. These plants often comprise substantial part of animal feed, thus presenting a potential source of phytoestrogens.The interest in these compounds was started earlier, over 50 years ago, when they have been known to affect fertility of an Australian sheep herd, resulting in an economic loss.  The putative effects of phytoestrogens are based on their structural similarity to the mammalian estrogen 17β-estradiol (E₂) and thus their potency to bind with mammalian estrogen receptors. More attention has been paid to these metabolites since they can act as estrogen agonists or antagonists which make their effects are not expected.. The estrogenic activity of phytoestrogens depends on many factors such as chemical structure of the compound, bioavailability, animal specie, responsive tissue and its estrogen receptor (ER) sub-type (ER_α or ER_β) and metabolites resulting from their fermentation and digestion. Reports showed that phytoestrogens could affect animal fertility and growth, quality of animal products and human health. This review illustrates the impacts of phytoestrogens on the livestock production system, focusing on physiology of reproduction, as well as the knowledge obtained from research in laboratory and field scales.