Tannins are naturally occuring plant polyphenols. The main characteristics of tannins are that they bind and precipitate proteins, they can have a large influence on the nutritive value of many food and foodstuff eaten by humans and animals. Especially in dry season trees and shrub foliages can provide significant protein supplement in animal nutrition but this feed resources are generally rich in antinutritional factors, particularly tannins. In this review; toxicity of tannins, their positive and negative effects on nutrition and detannification methods are evaluated.