Abstract
A review is presented of the physics that determines phase stability and structural transitions in close-packed metals. Microscopic considerations show that both long range (Friedel) electronic and short-range elastic contribution to the total energy break the degeneracy among different stacking polytypes. These factors largely determine the structural phase diagram as a function of band filling for both simple and transition metals. A phenomenological model based on the observation suggests that the HCP - > FCC transition should be regarded as continuous rather first order. The effect of external pressure, shear stress and temperature are explored in this context and contrasted with more traditional views. Examples are drawn from the elemental metals, alloys and artificial metallic superlattices.