Nineteenth-century science enabled us to design strong engineering structures such as bridges, boilers and dams. Twentieth-century science is enabling us to design the materials of construction themselves, such as super-alloys, new ceramics and fibre-reinforced composites. Engineering materials are no longer regarded as unfathomable ‘black boxes’ with immutable properties; we can probe into them, both experimentally using such tools as x-ray diffraction analysis and electron microscopy and also theoretically using such tools as atomic theory and statistical thermodynamics, to find out how their atoms are packed together, how these packings determine the bulk properties of the material and how to produce those packings that give the best properties. A11 this has transformed the subject known as ‘Strength of Materials’, once a branch of classical mechanics concerned with the design of enqineering structures, into a modern applied science in which atomic physics, molecular chemistry and physical metallurgy join forces with engineering and economics for designing strong materials according to clear scientific principles.