In this paper the place of the determination of the tool-chip contact length in a predictive analysis of orthogonal machining is presented and its relative importance to various aspects of machining is considered. The necessary accuracy of determination of the contact length is explored in relation to the predictions of a comprehensive thermal model of orthogonal cutting. Several 'theories’ for the prediction of contact length are reviewed and their predictive capabilities experimentally assessed. A number of these models include parameters which have to be established from experimental cutting tests; nevertheless most of the models provide qualitatively plausible trends in contact length prediction, although none can be considered sufficiently accurate from a quantitative point of view. It is shown that the most adequate method of contact length estimation is to curve fit multivariable regression equations to cutting data, and incorporate these in the basic cutting data base for use in predictive machining performance models.