The experimental and theoretical investigations of grain boundary diffusion processes have been performed using metals and alloys in nanostructured state produced by severe plastic deformation and the respective polycrystalline counterparts. The main features of diffusion-controlled mechanisms of plastic deformation observed by the creep of nanostructured metals are considered. The use of severe plastic deformation treatment and of the effect of activation of diffusion-controlled processes for enhancing the properties of nanostructured steels and alloys designed for engineering and medical applications (nanostructured titanium-bioactive coating composite included) is described and examples are offered.