Tool wear is discussed as a metallurgical problem. The presence or absence of a built-up edge (BUE) is important in relation to tool wear and surface finish. The possible mechanisms and processes of wear influenced by the different types of built-up edge are discussed. Metallographie investigations have shown that recrystallization occurs in the flowed layer. The flow of submicroscopic crystallites over the tool face influences tool wear. From the investigation, the most favourable cutting conditions, from the point of view of both wear and surface finish, occur in the absence of a built-up edge. © 1971.