Present-day technology makes greater and greater demands on the strength properties of materials. A physical theory of strength has not yet been developed, although the last 30 or 40 years have seen the development of a new branch of molecular physics which is concerned with the study of the deformation and fracture of solid materials. In this review we examine present physical ideas about the strength of crystalline materials, the reasons for the low strength of real materials, and also the main ways of increasing their strength.