The plant polyphenols are a very heterogeneous group, some universally and others widely distributed among plants, and often present in surprisingly high concentrations. There have been diverse speculations about their significance for the life of the plant. For present purposes, they are conveniently discussed astannins andnon-tannins. Tannin-protein reactions are important in the preparation and enjoyment of tea, wine and beer. Tannins interfere with the digestion of proteins in poultry, pigs and human beings, but sometimes may influence that process favourably in ruminants.Non-tannin polyphenols can undergo oxidation to semiquinones and quinones. These can undergo further oxidative polymerization, as well as coupling to proteins, by a wide variety of chemical reactions. Some of these reactions may serve to protect the plant against infections, parasites or predators. Such reactions, under the name ‘enzymic browning’, cause difficulties in food processing. More detailed study is required of possible damage to the nutritive value of proteins; this is particularly important for poultry and pig rations. It has become increasingly clear that these reactions of plant polyphenols contribute substantially to producing the organic matter of soils. Indeed, it is possible that this function has determined the evolutionary history of some of these polyphenols, and that they have no immediate function in the living plant. As the nature and quantity of polyphenols differ greatly between plant species, agricultural scientists should consider more seriously the effects of different crop plants and of different agricultural practices on soil organic matter.