Hypotheses concerning the diet of early hominids have played an important role in discussions on human evolution. Three investigations have helped define the extent to which dietary hypotheses may be taken and still be testable. Comparative anatomy is a fairly coarse approach, which despite convergences allows only the most specialized diets to be ruled out. A biomechanical analysis makes it clear that the changes in jaw and tooth form are subtle and outside the resolution given by present understanding of cranial function. Analysis of the microscopic tooth wear of extant species has been carried out. Major dietary types can be distinguished by their microwear. The microwear on fossil hominids appears to rule out certain diets that have been proposed for them.