A brief account is given of the origin and development of the electron theory and its successful application to improving physical and mechanical properties of a range of materials, including hard metals, metal-ceramic bonding, intermetallics and steels. As the nature of the bonding in solids, i.e. metallic, semi-conductor or insulator, determines both their mechanical and electronic properties, it is suggested that a theory linking both properties is desirable. The development of such a theory could be the basis of a better understanding of the mechanical properties of solids and the production of new materials for various technological applications.