In an effort to create structural "supermaterials", combining ultra-high strength, ductility and excellent fracture toughness, nanostructured materials are a promising strategy. Especially nanocrystalline metals, ultrafine-grained metals and metal-metal nanocomposites show great potential to break the mutual exclusivity of these mechanical properties. However, the vast amount of grain boundaries and interfaces within these materials typically act as weak links in the microstructure and limit any further enhancements in ductility and toughness. Thus, this work attempts to increase the strength of these weak links with the help of doping elements that were identified in ab-initio simulations to improve grain boundary- and interface cohesion. The two material systems selected for this approach are ultrafine-grained tungsten and nanocrystalline tungsten-copper composites, two materials with exciting implementation potential in future high-performance applications, such as nuclear fusion reactors. After introducing the theoretical concepts and state of the art in research, this work presents the developed fabrication and processing routes, involving powder compacting and severe plastic deformation, to create the undoped and doped materials. The influence of the doping elements on microstructure and mechanical properties is assessed using high-resolution characterization methods and small-scale testing techniques. Significant improvements of mechanical properties of either material system could be achieved through certain doping elements, while others had close to no or even a detrimental effect on mechanical performance. The different responses of the nanostructured materials to the various doping elements are discussed in detail. Finally, the effects of helium irradiation, as encountered in nuclear fusion reactors, on swelling and mechanical properties of the investigated material systems was characterized and compared to their conventionally structured counterparts, underlining their application potential in nuclear technology once more. Altogether, the strength-ductility trade-off could be challenged by applying grain boundary and interface doping to nanostructured metals. The overall mechanical performance of ultrafine-grained W and W-Cu nanocomposite could be enhanced greatly, rendering them viable options for employment in high-performance applications.