Abstract: Tannin compounds are widely distributed in many species of plants commonly consumed by ruminants, where these compounds play a role in protection from predation and perhaps as pesticides and in plant growth. Tannins are divided into two major groups and condensed hydrolysable tannins. One of problems of feeding agro-industries and agricultural by-products to farm animals are regarded to high content of anti-nutrients factors such as tannins and alkaloids. Tannins had a negative or positive effect on ruminants depending on the type of tannin consumed, amount ingested, chemical structure and molecular weight and the animal species involved. Low tannins concentrations may improve the digestive utilization of feed mainly due to a reduction in protein degradation in the rumen and a subsequent increase in amino acid flow to the small intestine. While, increase amounts of tannin concentrations in diets animals reduce feed intake, nutrient digestibility, milk production and animal performance. Addition of polyethylene glycol had beneficial effects in monogastrics and both beneficial and adverse effects in ruminants depend upon amount of tannins in the diet. Also, tannase enzyme can hydrolyze tannins substrates releasing gallic acid and glucose.