The concept of grain hardness still remains to be fully elucidated. It was often mistaken for vitreousness and even for strength of a flour. In fact, hardness essentially depends on genetic origine of wheats and is defined as the more or less friable characteristics of endosperm. Consequently, hardness strongly influences the milling behaviour of wheats as well as the yield in each milling fraction, although the yield in total flour is not associated with kernel hardness. By acting on the degree of disaggregation of particles, granulometry and starch damage, hardness primarily affects flour hydration, especially in low-hydration doughs. However, hardness does not influence flour strength, which remains mainly determined by the composition in storage proteins. Taking into account the world-wide market, it is highly recommended to include hardness in the system of wheat grading.