Tannin extracts have recently been found to be of value in protecting ferrous metals from both atmospheric and underground corrosion. The way in which their value in this field was discovered is discussed, and the present status of both applications is dealt with in detail. Used as wash-primers before painting, solutions of tannin extracts lay down an insoluble weather-resistant ferric tannate on a ferrous metal surface. The advantages of this approach over alternative techniques are described and the conditions for optimum results are discussed. The possibility of incorporating suitable tannins into emulsion paints to make the latter suitable for protection of exposed metals has also arisen from this work. In the field of underground metal corrosion tannins are of particular value in exhibiting a bacteriostatic effect against Vibrio desulphuricans, thus preventing the rapid corrosion of iron and steel pipes that this organism causes. Field trials are now under way to assess the practical potential of this observation. Finally, it appears likely that the tannins may also be of value in protecting non-ferrous metals. -- CCI