Chemical substances from the group of phenols (polyhydroxyphenols), which are found in the barks of many trees, perennials and shrubs as well as in the skins of fruits and also in the grapes, especially of red wine varieties. They ultimately serve the plants as a defence against herbivores or predators and also as protection against microorganisms. Tannins can inactivate a variety of viruses and therefore have an antitoxic effect. They also have a very strong tanning effect, which is why they have been used for thousands of years in the production of leather. For this reason, they are often colloquially referred to as tanning agents, although this term has a rather negative or pejorative connotation compared to tannins.