Tooth wear records valuable information on diet and methods of food preparation in prehistoric populations or extinct species. In this study, samples of modern and prehistoric hunter-gatherers and agriculturalists are used to test the hypothesis that there are systematic differences in patterns of tooth wear related to major differences in subsistence and food preparation. Flatness of molar wear is compared for five groups of hunter-gatherers (N=298) and five groups of early agriculturalists (N=365). Hunter-gatherers are predicted to develop flatter molar wear due to the mastication of tough and fibrous foods, whereas agriculturalists should develop oblique molar wear due to an increase in the proportion of ground and prepared food in the diet. A method is presented for the quantitative measurement and analysis of flatness of molar wear. Comparisons of wear plane angle are made between teeth matched for the same rate of wear. Agriculturalists develop highly angled occlusal wear planes on the entire molar dentition. Their wear plane angles tend to exceed hunter-gatherers by about 10° in advanced wear. Wear plane angles are similar within subsistence divisions despite regional differences in particular foods. This approach can be used to provide supporting evidence of change in human subsistence and to test dietary hypotheses in hominoid evolution.