Although the nineties witnessed an intensity of research on nanocrystalline metals owing to their ultra-high strength, the early enthusiasm was met with severe disappointment due to their brittle nature and loss of structural stability. In contrast, research over the past few years has provided compelling evidence that nanotwinned structures may be the optimal motifs for the design of both high-strength high-ductility materials. Indeed, nanotwinned Cu containing twin lamella of 20-100 nm thickness has been shown to possess a yield strength in excess of 1 GPa with elongation to failure as high as 14%, which is in sharp contrast to nanocrystalline Cu having a yield strength of about 400 MPa and elongation to failure of about 2-3% for comparable grain sizes [1].