Nanostructuring of metals and alloys by severe plastic deformation techniques is an effective way of enhancing their mechanical and functional properties. The features of the nanostructured materials produced by severe plastic deformation are stipulated by forming of ultrafine-sized grains as well as by the state of grain boundaries. The concept of grain boundary engineering of ultrafine-grained metals and alloys is developed for enhancement of their properties by tailoring grain boundaries of different types (low-angle and high-angle ones, special and random, equilibrium and nonequilibrium) and formation of grain boundary segregations and precipitations by severe plastic deformation processing. In this article, using this approach and varying regimes and routes of severe plastic deformation processing, we show for several light alloys (Al and Ti) the ability to produce ultrafine-grained materials with different grain boundaries, and this can have a drastic effect on the mechanical behavior of the processed materials. This article demonstrates also several new examples of attaining superior strength and ductility as well as enhanced superplasticity at low temperatures and high strain rates in various ultrafine-grained metals and alloys.