Inspection of the scientific literature shows that there are many techniques that can be used to produce nanostructured materials, including inert gas condensation or chemical vapor condensation, pulse electron deposition, plasma synthesis, crystallization of amorphous solids, severe plastic deformation, mechanical alloying or cryomilling. However, only a few of these techniques, such as equal-channel angular pressing (grain sizes 200 - 1000 nm), electro-deposition and cryomilling (grain sizes 30 - 500 nm), generate nanostructures with sufficient thermal stability to permit the fabrication of bulk materials. In the present work, nanocrystalline materials were produced by mechanical attrition under liquid nitrogen (i.e., cryomilling). The grain refinement process was dominated by the total microstrain introduced by the deformation process. The microstructures were investigated in detail using transmission electron microscopy and high- resolution electron microscopy. Three nanostructures with different grain size ranges and shapes were observed and the deformation mechanisms in these structures were found to be different. High densities of dislocations were found in large crystallites, implying that dislocation slip is the dominant deformation mechanism. The dislocations rearranged to form small angle sub-boundaries upon further deformation, resulting in the formation of medium-sized crystallites with diameters of 10 - 30 nm. In very small crystallites with dimensions less than 10 nm, twining becomes an important deformation mechanism. Some defects, such as twin boundaries, and small- and large-angle grain boundaries were investigated in detail. Both non-equilibrium and equilibrium grain boundaries were found to exist in the cryogenic ball milled powders. The grain growth kinetics in the nanocrystalline Al and Al-Mg exhibits extremely high resistance against grain growth at elevated temperatures. Tensile behavior of bulk nanostructured Al alloys consolidated by cryomilled powders was characterized by high strength, high ductility and low strain hardening. The present lecture will also address the hypothesis that one can promote dislocation activity.