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. The amount of tannins and their effects on animals range from beneficial to toxicity and death. In this review, chemical structure of tannins, plant species containing tannins, their pharmacological effects and the analysing methods of tannins are discussed.