Temperature gradients in high speed steel tools used to turn iron, titanium and nickel have been measured using a method based on the structural changes in high speed steel heated to temperatures over 560°C. The relationship between temperature gradient, cutting speed and feed rate have been demonstrated through the heat affected region close to the cutting edge. The most emphatic feature of the results is that each of the three metals imposed its own and greatly different pattern of temperature distribution within the tool. It was shown that this controls some of the basic wear processes which limit the rates of metal removal. When cutting iron there was a relatively cool zone close to the cutting edge in the most highly stressed region, while the hottest part was well back from the edge; the temperature gradients were very Bteep. When cutting titanium the higher temperatures were much closer to the cutting edge, and the total heated region was smaller, while the temperature gradients were again very steep. When cutting nickel the tool edge was at least as hot as elsewhere in the heat affected area, and temperature gradients.near the edge were much less steep. The results have Televance to the understanding of tool wear, the design of cutting tools and the application of cutting lubricants.