Summary A review of the radiometric evidence suggests that the top of the Devonian should be dated at about 345 m.y. and the base of the system at about 395 m.y. Using cumulative thickness methods the age of the base of the Upper Devonian is estimated at 358 m.y. and that of the base of the Middle Devonian at 370 m.y. Famennian ammonoid species zones must each represent about a million years. Suggestions are made as to where future radiometric work of importance to the time-scale might be undertaken.