Wheat grain milling has for aim to isolate the smaller starchy endosperm particles (i.e. flours, semolina) from the larger fragments of peripheral tissues. The mechanical properties of the wheat grain tissues strongly influence how effective the steps of grinding and sieving are during this fractionation process. The grain mechanical resistance determines how much energy is required to fracture it, the particle size of the resulting products, and their biochemical composition. Therefore mechanical properties affect both the durability and the quality of the processed products. Genetic loci, and more precisely the key role of the Hardness (Ha) locus in the D genome of common wheat (Triticum aestivum), are well established determinants of the mechanical properties and behavior of grain, which are also influenced by environmental factors. The key role of genes encoding puroindolines has been confirmed by extensive analysis of mutants and through genetic manipulation. Methods of measuring mechanical resistance are being reconsidered because grain hardness needs to be characterized in ways that capture the different contributions of genetic and environmental factors. In particular, methods to acquire data on the mechanical resistance of each of the grain tissues and their components have been developed. Finally, the promise of using numerical modelling to better understand and predict the effect of changes in the wheat starchy endosperm composition will be discussed