Deformation twins and stacking faults have been observed in nanostructured Al-Mg alloys subjected to severe plastic deformation. These observations are surprising because deformation twinning has never been observed in their coarse-grained counterparts under normal conditions. This paper reviews experimental evidences on non-equilibrium grain boundaries, deformation twinning and partial dislocation emissions from grain boundaries. Some of these features can be explained by the results reported from molecular-dynamics simulations of pure fee metals. Special emphasis is laid on the recent observations of high density hexagonal and rhombic shaped nanostructures with an average size of 3 nm in the Al-Mg alloys processed by high pressure torsion. A possible formation process of these nanostructures are proposed based on molecular-dynamics simulations. © 2009 Materials Research Society.