Tannins are a group of phenolic compounds which have received a lot of attention with respect to their possible nutritional and physiological actions. Tannins can be grouped into condensed and hydrolysable tannins, widely distributed in the plant kingdom. Hydrolysable tannins are polyester of phenolic acids such as gallic acid or ellagic acid or their derivatives and D-glucose. The condensed tannins are polymer of flavan-3-ols, flavan-3,4-diols or related flavanol residues linked via carbon–carbon bonds. There is evidence that high levels of tannins could produce adverse effects in animals. These effects can be instantaneous like astringency or a bitter or unpleasant taste or can have a delayed response related to antinutritional/toxic effects.